#1 Lio Rush - 24 years old

Lio Rush made his first televised WWE debut on an episode of NXT back in 2017 where he was attacked by The Velveteen Dream, his former tag team partner. He was moved to WWE 205 Live where he competed and was known as "The Man of the Hour." He made his first televised debut as a heel on an episode of 205 Live in June 2018.
Lio Rush was placed on Monday Night Raw where he was the manager of former ECW Champion Bobby Lashley on Raw. He has also had a few matches on the A-show and even defeated Elias. Rush is still employed by WWE today even though he hasn't appeared on TV for a while.
At the age of 24, Lio Rush is currently the youngest WWE Superstar on the main roster.
Also Read: 10 Youngest female WWE Superstars on the current roster
Becky Lynch has been challenged HERE.