In the wild it comes down to luck, & I'm pretty sure that many snakes wouldn't turn down small frequent meals- I just don't think many get the option.
Breeding is much easier on the males body than on females. I personally never bred any snakes younger than 4 years...it only makes sense for the female's body to be bigger & better developed before producing eggs (or having live neonates, for those species that do). I believe Wilbanks said he waits 2-3 years at least, which is typical for those in the business.