If you are trying to 1-position train, I recommend going with 2 players to save you the risk of having someone not getting as much training as the others, but you can go to 3 people. If you 2-position train, you can get up to 5 people, but this way of training only gets you 75-80% of what 1-position training does. It is better to focus on a set of players. Switching off between different people causes you to lose money in the long run. The best age to start training is 18 YO, but 19 is also fine. If you try to train, for example, a 25 YO, you probably want it to be an MVP or HOF potential player that you are trying to finish off. A bad age to train would be like an 24 YO with allstar potential. He wouldn't get very many pops due to age, and if he is at or close to his cap, because he is pretty much at his capacity.