Most shonen anime is geared toward younger men, particularly adolescents and teenagers. As a result, it's no surprise that the protagonist's age reflects this.
With some protagonists as young as 10 and others as old as 28, it's worth considering how young some shonen anime protagonists are when they begin their journey because it helps to ground the viewer when they have to make difficult decisions or otherwise embark on these perilous quests.
This list will chronicle the 10 youngest shonen anime male protagonists, ranked from oldest to youngest.
Disclaimer: This list will contain spoilers regarding various shonen anime. It only reflects the author's opinion.
Rule 1: The character's age must be proportional to where they began in their respective anime/manga.
Rule 2: If two characters share an age, then their manga's start date will be taken into account.
The 10 youngest shonen anime males, ranked from oldest to youngest
1) Kenshin Himura (28)
Kenshin Himura, a rare example of an adult protagonist in a shonen anime, has seen and done a lot in his 28 years of life. He's killed people, fought in wars, and yearned for a life of peace and atonement after witnessing and dealing with so much death. This is not easy, as many people from his time still hold grudges against him for his man-slaying days.
Despite this, Kenshin vowed never to kill again. This was tested during the events of Rurouni Kenshin. It's something of a miracle that he's able to settle down with a family at the age of 33, given the sheer amount of fighting he had to do to get to that point. Finally reaching that goal of a peaceful life was indeed something precious.
His life with his family would be covered in other material.
2) Jotaro Kujo (17)
Coming off a good decade behind Kenshin is Jotaro Kujo in the long-running shonen anime Jojo's Bizarre Adventure, specifically Stardust Crusaders. Arguably one of the most iconic of the Joestar family line, Jotaro does quite a lot for a 17 year old. From going on a trip to Cairo, Egypt, from Japan to killing the reborn DIO, Jotaro has seen and done much in just 50 days of in universe time.
Jotaro was already a delinquent by the start of his role in the series, already facing self-imposed jail time for beating down four gangsters. It was self-imposed because he believed he was possessed by a demon. After learning about Stands and their uses, he started the journey with the Stardust Crusaders.
Jotaro would later be the only one alive by the time of Stone Ocean, at the age of 40, if Polnaraff's soul in a turtle doesn't count.
3) Ichigo Kurosaki (15)
Just a couple of years shy and well into teenagehood is Ichigo Kurosaki of the shonen anime Bleach. Despite a generally surly attitude, Ichigo is not as delinquent as Jotaro is. Ichigo kept up with his schoolwork and usually received top marks in class...before and after encountering Rukia and being dragged into the Shinigami life, that is.
From the day Ichigo encountered Rukia, he was dragged into the life of a shinigami. Despite only being a substitute, he still performed his job admirably by purifying Hollows. Flash forward an arc or two, during which he and his friends had to raid Soul Society to save Rukia, and Ichigo was made into an official shinigami.
This status is still on him as of the manga's end, where Ichigo was 29 and married to Orihime.
4) Izuku midoriya (14)
Born powerless in a world of superhumans, young Izuku Midoriya always dreamed of being a hero like the Symbol of Peace All Might. He got his wish at the age of 14, when All Might started passing down his Quirk One for All to him. Granted, it took him 10 months worth of hard training just to get ready for the entrance exams, but he eventually started at U.A. High School.
This plucky, nerdy hero-in-training has already seen combat at school and during the USJ Incident. The big problem with One for All, at least at the start of the shonen anime My Hero Academia, was the injuries that Izuku inflicted upon himself when using it. Flash forward to the current final arc in the manga, and Izuku has not only overcome that limitation but unlocked multiple other Quirks and is embattled with the series' main villain All for One.
Izuku is now a far cry from when he started, able to balance out all of his Quirks much easier now.
5) Tanjiro Kamada (13)
Born into a peaceful life outside of a village, Tanjiro Kamada's peace was shattered when the demon lord Muzan targeted his family and slaughtered most of them. Only his sister Nezuko survived, and thus began a long and hard journey across the world to cure her of her demonism. Tanjiro was only 13 at the time, and this trauma followed him all the way through to Muzan's defeat.
To put the shonen anime Demon Slayer into basic form, it's Tanjiro and Nezuko's journey to not only find a cure but to save people from demons run amok. It was rather difficult to start out, as the two nearly lost their lives against a demon hunter who only saw Nezuko as a demon to be slain.
Flash forward towards the end of the series, however, and Tanjiro and his allies face down and eventually kill Muzan.
6) Naruto Uzumaki (12)
The future Hokage of Konoha started off as an ostracized kid, pulling pranks and wrecking things as he went. It's not all that surprising that Naruto Uzumaki, the titular character of the shonen anime, would start out like this given his backstory. Naruto was fused with a monster called the Nine-Tailed Fox which ravaged Konoha, with it sealed inside of him.
To that end, Naruto received plenty of ostracization and condemnation by society. He vowed to prove them wrong, not only by becoming a ninja but by becoming Hokage. This plucky ninja ended up proving himself more than once during the course of the original series, from besting foes like Neji and Gaara, to perfecting the Rasengan and ending the Shinobi World War at the age of 17.
Naruto eventually became Hokage at the age of approximately 25, and had a family with Hinata Hyuga.
7) Son Goku (12)
While most fans may see Son Goku as a 24 year old man in the shonen anime Dragon Ball Z, the fact is he started out at the age of 12. A comedic mistake made by Goku himself was that he thought he was 14, but never learned how to count. Despite Goku's diminutive stature, he thoroughly proved Yoda's old adage that "size matters not" multiple times.
Goku was already able to withstand powerful blows and keep going long before his Saiyan heritage was ever revealed. Not only did he survive plenty of punishment, from beatdowns to boulders to fighting giants, but he could also dish it out with his power pole and kamehameha technique. Goku defeated King Piccolo and Piccolo Jr., laid waste to the Red Ribbon army, and much more as a youngster.
His exploits as a fully grown, if not entirely mature, adult have already reached godly status as of the shonen anime sequel series Dragon Ball Super.
8) Taichi "Tai" Kamiya (11)
A summer camp trip gone awry saw several children get trapped inside a "digital world" and destined to save it alongside their Digimon. Taichi "Tai" Kamiya was barely 11 years old when he was summoned, and dubbed himself the leader of the band of kids. Over the course of the original shonen anime Digimon Adventure, Tai would learn that leadership isn't just about getting people to follow your lead. His rivalry with Matt would be a huge test of his skills.
It's just scratching the surface of the events of the shonen anime Digimon Adventure, which started the anime franchise in earnest. Tai himself would go on to not only defeat Myotismon multiple times alongside the others, but other larger threats throughout his life.
To round this off, he apparently became a United Nations diplomat to the digital world as an adult and even had a son.
9) Ash Ketchum (10)
Putting aside the "Forever 10" jokes that are doubtlessly made at Ash Ketchum's expense, this shonen anime Pokémon trainer started off with just a rather irritable and disobedient Pikachu. While Ash hasn't entirely aged since the start of his journey, it's worth noting just how much he's accomplished since he first started off.
From capturing many Pokémon to nearly completing the Pokédex and being around many legendaries, Ash has seen a lot since he started in Kanto. From beating a ton of gym leaders to facing off in tournaments, he didn't exactly have a good win streak but always managed to do better than he did in the league prior.
Finally, after a whopping 21 years, he became Pokémon champion during the Alola Manalo Conference in Sun and Moon. He also earned the distinction of being that conference's first champion ever.
10) Conan Edogawa (~6-7 years old)
Shinchi Kudo was a 16-17 year old up-and-coming detective until he was ambushed and drugged with a chemical compound known as APTX 4869. Instead of killing Shinchi as it should, however, it regressed him back to the body of his six-to-seven-year-old self.
The newly dubbed Conan Edogawa has spent the majority of the long-running Shonen anime detective series Detective Conan solving murders. While Conan might be a child, his intellect remains as sharp as it was when he was an adult. This manifests in his problem solving and skills of deduction when on a case.
Though still unable to fully expose the Black Organization that drugged him, Conan is assured he'll get them eventually.