It is easiest to earn income at level 14~15, before player market value is capped. After that, increased wage take over and you will be unable to raise much money from training young players. At the lower levels, young players generate net profit thanks to increased market value duing the time he play and train in your team.

Unless you play the game for over a year and reach level 20 beyond, income is easy to come by and you can max out all buildings rather easily.