that all depends on their work ethic, natural ability and if they actually have a plan.
if they didn't come of age in high school, it aint the end of the world because a lot of colleges have open tryouts to scoop up walk-ons... or there is also going to a comm or junior college like someone else said.
If you didn't go to college or didnt play there or just wasnt that good, then shyts tougher but the DLeague stays having open tryouts in the major cities as well. Its bottom of the barrel shyt of course but, at the end of the day, if you can play.. and i don't mean you the man at your local bullshyt YMCA and no where else... but that you legit got a jumper, handle, and decent height (6'2+), size and/or athletecism then you'll have a chance up until your mid-20s.
but not too many people who talk that big talk have a legit jumper, handle, and the height/size/etc to even be recognized outside the 'system' to begin with... and the few who do, either gotta work a 9 to 5, got baby mommas, smoke/drink too much or on some other type of bullshyt that they aint working hard for it anyway.
I had one homeboy a year or so ago who made it overseas in one of them leagues despite not playin on a college squad outside of travel teams and intramural leagues at our university.. but dude was a freak of nature athletically, had legit skills to begin with, and put in serious work in the gym during the semesters and summers. From what I understood, he either went to one of those Dleague tryouts and got seen or he got discovered in one of those city-teams that he tried out for.
either way, he was a rare case of height (6'4), talent, athletic ability and a perfrect situation to continue working on his game (prob either student loans or living with his folks)... despite being outside of the traditional 'system'. The vast majority don't have all those factors going for them to be realistic with their "dreams" however.
I say its well even trade 
