You are right!
Old players in your team now will destroy your idea. I said It's necessary to have a good training, Ralph had players in U21 NZ and now he has a stronger team because he did a good training plan.
You need patience in this game, you need to buy 2 or 3 18 yr players, and 3 season you'll have 3 good players. Actually I have an U21 International Player in Brazil, 3 season doing a good job, now he's the best player in my roster.
Improve your stadium should be good too, but first you need to build a training plan, so in the future you will see how better your team will be.