Two points you must note. First, not all young players are fast trainers and develop well enough to play for many seasons. So you must find the right one to develop. Second, you need extra tokens, rest packs and even red packs to power train young players to accelerate their development.

If you are not token buyer or farmer, the general strategy may be buying players you can afford, using them for about 3 seasons (one contract period), and then replacing them with high quality players. Depending on your resources (mainly cash), you may use a shorter cycles. If you buy young players, you may use them for longer period.

Different teams face different environment (different transfer market, different competitors). You should find the right strategy for your team.

When my teams were in very low level, a 3* or 4* team was sufficient to compete for league titles. My young players developed very rapidly. So I could use many players for 6 or 7 seasons. Now I need about 5* team to compete. Most of my players are needed to replaced within 2 seasons.