Is it best to buy the expensive players?

I would think that the young players (18-23 years old) who are expensive can develop much faster and become great players. I'm a level 5 and the average price for a 18-20 year old who is a 4 star or 5 star is about 3-4 million which is the most expensive. I bought a 23 year old player whose quality is 44 (5 star player) for about 3.3 million and I trained him at the highest to see how much he would develop and he developed very little, which made me mad because I wasted 3.3 million on a player who, I thought, would develop fast.

it is good getting the expensive one`s but the young ones as they train faster than the older one i find they slow at 22/23 yr old

There are age brackets which decide training, 18-21, 22-25, 26-29, 30+, or something like that. The older the bracket the slower they train.