To make all your players increase to over 90 in two or three seasons, before you start the season, go to the "Team Management" menu. Select "Edit Players" then select the team you want to use in Manager mode. Edit all the players and decrease their age to the minimum (18 years old) setting their year of birth to 1990. Then, select "Save and exit" to save the changed data. Start Manager mode with the team you selected earlier. Young player will grow a lot with match ex. and grow two to five levels after every season if they play well.