4. Roberto Osuna
The Mexican-born right-handed pitcher was signed by the Blue Jays as a 16-year old in 2011. Although there were fears that Roberto Osuna might fight with his weight as he progressed. Just two months after his 20th birthday he made his major league debut in 2015 and quickly became the most dependable arm in the bullpen, which led him to being the closer.
In his rookie season, Osuna finished fourth in Rookie of the Year voting when he collected 20 saves and had a 2.58 ERA. Osuna would become an All-Star by the time he was 22-years old and is currently third All-Time in saves by a Toronto Blue Jay. He would have likely passed Tom Henke had he not run into legal trouble in 2018 that led him to being traded to the Houston Astros.
Since being traded to Houston, Osuna has led the league in saves in 2019 with 38 and already has 154 over his career while still being just 25 years old. By comparison, Marian Rivera never picked up his first save until he was 26-years old.
3. Vladimir Guerrero Jr
The Blue Jays signed Vladimir Guerrero Jr as a 16-year old in July of 2015 for $3.9 Million and he has been arguably the most hyped prospect in team history. VGJ excelled through parts of four years in the minor leagues and played in his first game last season shortly after his 20th birthday. He played in 123 games in his rookie season that saw him hit 43 extra base hits with a .272 average which had him finish sixth in Rookie of the Year votes.
While he has only one season of major league time under his belt, the hype looks to be accurate. It is very rare that a player can crack a major league roster at 20-years of age, Guerrero Jr was able to do it at a high level. It will be exciting to see what Guerrero Jr has in store for Blue Jays fans for the next decade.