I think about this a lot when formulating a counterargument to "Federer is past his prime." My argument goes...
(1) the game and competition is much different today than when it was in 2005 and 2006, and (2) he's arguably playing the best tennis of his life. In fact, the latter can be said of several players 28 and older.
I know it mollifies the injury when a Fedfan says, "He's not dominating because he's past his prime," but I'm not buying. Because of the competition and change in surfaces, his game has had to evolve. Today, surfaces favor terrible tennis* -- ask "Killer" [LOL at the irony] Cahill and Patrick McEnroe, the ESPN boys are the biggest cheerleaders of terrible tennis -- but that's conflating evolution (bad) with Roger's age.
As for the young guns, it's hard to set expectations high today. Our expectations have been high for years, and all that's made us is disappointed.
*terrible tennis def. stand well beyond the baseline, wait for your opponent's UE or drop from fatigue, play high % shots, i.e. don't strike first
best (i.e. worst) example: Djokovic-Murray
others: any combination of Djokovic, Murray, and Nadal
Terrific tennis: 2013 2R Wimbledon, Stakhovsky def. Federer 6-7 (5), 7-6 (5), 7-5, 7-6 (5)