many dont have official amateur careers- like manny and mexicans, etc, who all turned pro early, but almost all pro boxer started young - that i can think of. it takes 10 years i reckon to develop the body, the skills to be good at a sport like boxing. mundine started in his late 20's but his dad was a pro - and he d been boxing for fitness as a kid.
Scotland's second best fighter, middleweight Bert Gilroy never fought amatuer - 15 years old straight into the Pro's.
If you're a quick learner, discipline and dedicated you dont need to fight in the amaturs since with the way amateurs are scored nowadays they are two different type of boxing. You can take a 16 or 17 yr old kid teach him the basic spar him in the gym. And put him straight into four rounders with low level club fighters where he'll learn and gain the experience he needs. :hat
Manny Pacquiao had 64 amateur fights (60-4) and was on the Philippine National Amateur Boxing team http://www.philstar.com/Article.aspx?articleid=649126 Sergio Martinez did not start boxing until age 20, but he had 41 amateur fights. http://ringtv.craveonline.com/blog/124735-sergio-martinez-better-late-than-never
George Foreman has , by far the shortest amateur career with the best success only 22 wins and a gold medal that is truley amazing !!
he had amateur fights in between pro fights as the rules were different way back then also quite a few people believe he also boxed under different names and does indeed have a few pro losses !!