#1 Brock Lesnar - 25 years old (WWE WrestleMania XIX vs Kurt Angle)
Brock Lesnar's WWE career has been historic for a plethora of reasons. The Beast made his WWE debut the night after WrestleMania X8 in 2002 on Monday Night RAW.
Lesnar quickly rose to prominence in WWE, defeating a variety of high profile WWE Superstars. After winning the 2002 King of the Ring tournament, Brock Lesnar earned an Undisputed WWE Championship opportunity at SummerSlam 2002 against The Rock.
After defeating The Rock at SummerSlam to become the Undisputed WWE Champion, Brock Lesnar broke records by becoming the youngest WWE Champion in history at the age of 25. Lesnar also became the second fastest WWE Superstar to win the WWE Championship after his debut at 126 days.
After dropping the WWE Championship later that year and turning babyface, Brock Lesnar eventually won the 2003 Royal Rumble match. This earned Lesnar a WWE Championship match against WWE Champion Kurt Angle at WrestleMania XIX.
Brock Lesnar and Kurt Angle both had the honor of main eventing WrestleMania for the first time in their careers. In the closing moments of the match, Lesnar infamously attempted to hit a Shooting Star Press but botched the move and landed on his head and neck. This caused Brock Lesnar to legitimately suffer a concussion.
Kurt Angle, who was set to have surgery on a broken neck, had to improvise a finish to the match. Lesnar was able to get his wits about him and hit Angle with an F5 to win the match and regain the WWE Championship once again.
By competing in the main event of WrestleMania XIX, Brock Lesnar once again made WWE history by becoming the youngest WWE Superstar to main event a WrestleMania pay-per-view at the age of 25.