Players with exceptionally high market value train faster.

Youngers players are more expansive in general. At the same quality, young players that train extremely fast are the most expansive. These known as fast-trainers are also the most sought after, often result in bidding wars.

At the lower level you should try to get as many young players as you can get. They will increase in value faster than the money you spend for him. But be warned as bidding war may happen, so getting a fast trainer can be a risky business.