Hasim Rahman started boxing at age 20 and just two years later he had his pro debut, and two more years later he already fought Tua for the IBF and did well while it lasted. Rather impressive if you think about it this way.
Steve USS Cunningham He didn't pick up boxing until after he enlisted in the Navy and didn't have his 1st amateur fight until he was 19 years old.
They say he was shadowboxing at a win Dixie grocery store. He was working the night shift throwing punches to keep himself awake, when a co worker noticed them handz & recommended a boxing gym. I believe Campbell was 24. Charles Martin started at 18 or 19 & became champ
Valuev started boxing at 19 and went 48:0 before his first MD loss. I noticed most late starters are HW. I don't know any lighter weight fighters, who started at 20 and won titles. Only Martinez?
Foreman started when he was 17, won the Olympic Gold at 19. Heavyweight title at 24. Also Rocky Marciano started very late.
I think that's normal as we get older. I'm still surprised at being older than most athletes (I'm in back half of my 30's), after they seemed like old men when I was a kid and a teen, lol.