Well, people have touched on it, but I think that the 18 year old season can be worth 8-12 skill points (depending on trainer level and whether you are filling holes). So, for a 19 year old to be worth it to me, he would have to have at least 10 more skill points than the 18 year old. Of course, you don't know tsp in the draft, so my draft board tends to be 4 and 5 ball 18 year olds sorted by potential, then 5 and 4 ball 19 year olds (I tend not to waste the draft points to interview them) followed by 3 ball players, unknown players, and, finally, 1-2 ball players.
Honestly, I would always start with an 18 year old as your primary trainee. If you want to have a 19 year old as a second or third trainee, I could see that, but they would really need to be above most 18 year olds. I'm talking like 60+ starting TSP. I think you could train a 60 TSP 19 year old over a 35 TSP 18 year old.