Well with the new transfer system it doesn't seem to be that much of a difference in token price between old and young players so try to go for the younger ones and only fight for the good older player if you don't see that many bidders.
Consider signing early in the next season 18yo around 85% as they can grow faster and be better than a good old player in a matter of days, plus not a lot of people bid on them since they are focused on over 90% players.