How many skills do you know, and what's the total difference? If there's more than 5 points difference in all skills (or the same ration for fewer skills if you don't know all, for example 3 points over 6 skills), I'd go for the 19yo. The 18yo would then only have a couple more points when he's 19, so I'd rather have more potential than a couple more skillpoints (unless you're gunning for U21). It's also valuable to not need to train when the trainee has low skill points so you have a better player for your games.