if you follow a players normal development trajectory, you will notice that most of them peak between 25-28 years of age. Iturbe is 21, is looking promising and is far from the final product. Cerci who is 26 is quite good but chances are that he is pretty close to the final product - only thing missing is playing for a big club.
I have seen both of them play, and they are both pretty good. I'd be happy with either but I prefer Iturbe.
Its not about peaking. After 25 years of age, a player has decent to good top flight experience ( considering he is a player of top flight quality at age 21 )
This added experience matures them as player and person, along with being in the most ideal physical shape. With good form at 26, we consider players to peak.
But its a very qualitative matter. Some players show great skill and confidence in their youth but over time that flair fades as they learn to play with a team since in youth level most promising players are the sole stars of their team which they carry forward in competitions. As one starts playing regularly in a team they learn to develop all round which in most cases takes away a certain edge that made them promising.
That is the average scenario for any players.
Exceptions and super stars are the ones who grow on all fronts and leave nothing behind. Ex. C. Ronaldo was a dribbling, skill-showing nut head who matured into a more attacking player.
A 21 year old who had 25+ goals in a season might earn himself a spot on an A-list squad. By 26, most of these players turn into 15+ goal scorers. They lose some aspect of their flair and gain a little more team-oriented style of play.
Signing 21 year olds for crazy sums is just like trading in futures. High risk, high gain.
