Do you mean which is faster? Or do u mean what age should u train passing?
Single position training is the fastest a player can train. Training is affected by height, age and trainer level and of course minutes (get 48+ every week)
Skills are always going up if they are trained. Once players get old like 29 they start to slowly decrease skills. The decrease will became notice and start popping down depending on sublevels. Thats why you dont see 30yo getting trained, because the decrease is more than the increase