Martinez has always been into sports though hasn't he? Soccer, bicycling and a lot of other stuff, not to mention he's incredibly athletically gifted.
19 is probably too late, most of the top pros have been doing it since they where old enough to have gloves laced onto their hands. They've lived and breathed boxing their whole lives. There are some exceptions though. Froch boxed for a long time as an amtuer but turned pro late, Hopkins got into it late, Martinez, think Maidana did aswell.