Players start to decrease in stats quite fast, in football manager.
Pace/acceleration will start decreasing as soon as the player hits 30, I think.
Good on you for signing Dzeko, though, I also took him in free, in the fourth season
You can't really use those "peak"-ages, though, since it all just depends on stats. You get two players with equal stats, they are going to (if both are exactly the same, except for their age, 21/29-yr for instance) perform exactly the same.
Only argument there is, is that older player will be more composed, but you can just look at a players mental stats to figure those out.
My advice is, get rid of Dzeko before he turns 30 - you're signing him on a free, and if he is a success, you can sell him for maybe 20 million next season. If you wait until, for example, 31, you can't sell him for anything proper anymore.
Strikers are a dime a dozen, to be honest, I'd be more reluctant to sell if you were talking about a quality left back, or something to the like
