Yeah, I'd agree with that, generally players are at their peak between 26-29. But it can vary. Defenders for example can often continue to play at a high level when they are older, as can players who don't rely on their physicality.
Of course players that rely on physical attributes can decline earlier. This is the context I look at when I consider Cristiano. Once you hit 28, especially as a forward that's the point when you can start declining, sometimes quite rapidly, it depends on the individual. In fact, it may have already started, since he scored 34 league goals last season whereas in the previous season he got 46(although he did improve in the cups). Maybe he will improve a bit, but at 29 it's extremely unlikely he'll improve a lot. It's likely we've seen the best Cristiano the season before last, and I think this is his last season to play at his peak. And the decline, might be slow, a couple of percent a year, but it's still a reduction in performance.
In terms of Iniesta, you can't make a direct comparison because Iniesta is a midfielder and so pace is less important. I wouldn't be surprised if he starts declining a bit in his dribbling soon, but he can make up for that with his positioning and passing, just like Xavi did at that age.
My point is that you can't say that just because Barca turned things around in the past another team has a strong chance to turn it around now. The most likely thing is a turnaround won't happen. It did in the case of Barca, but it needed a lot of things to change. And, at the time it was predictable that the change could be rapid as the potential for Messi, Xavi, Iniesta and Eto'o was clear for any Barca fan to see. But that requires a look at the detail. That is what my predictions are about.
Isco and Illara might make an impact, but the most likely thing considering they are only 21 is that they won't. They aren't competing in a normal league, they are competing against a team that has just won the league by a record margin. This demands a high level of consistency, something that is harder to attain at a young age.
Higuain struggled in CL, but RM probably won't face Barca in the CL. I'm talking about the direct competition in the league, this is where losing Higuain is a big blow.
Varane, Ramos and Pepe might be good, but two injuries and they face the same problem we had this season.
As for Puyol, Xavi and co. They played at a level that got Barca 100 points. They don't need to make an impact. They just need to maintain their level. Real Madrid need to improve. But their two best players at that age those two players won't improve, Cristiano hitting 29 as an attacker, and Xabi post 30 as a midfielder, more likely decline a little. At Barca the two best players; Messi and he is 26 so he will likely improve. Iniesta at 29 in midfield he should be able to maintain his standard and maybe improve a little. He also doesn't have heavy defensive duties like Xabi that need physicality.
Looking at the rest of the Madrid squad, I don't really see too much potential. Benzema and Varane I think are the two players with a lot of potential upside, especially Varane. Maybe Di Maria to some extent and Carvajal has a chance to improve right back. But Benzema lacks discipline, Varane might get figured out, Carvajal might flop and Di Maria lacks character.
If they get Bale, then yes he could make an impact. Suarez too if they went for him. Falcao was perfect for them but they missed out on him. And Lewandowski seemingly only wants Bayern. But I do think they need something.