aggessiveness cannot be trained, and the u24 bonus is 0,02 per update on every skill.
per season = 91 x 0,02 = 1,82 point
the total of all youth seasons = 5 x 1,82 = 9,1 points
so, the aggressivess skill is 9,1 points more at the 24th birthday, than what a 19yo has at the start of this season.
talent rating increases the training speed, it does not have effect on the u24 bonus.