It's very strange isn't it, and there are loads of old players getting their best ever results and I wonder why that is. A few possible reasons that I can think of;
1. is that due to sports science and nutritionists and the like players are probably more in tune to what they need to do to keep themselves healthy and going strong for longer.
2. The young generation isn't good enough - we've all heard this and probably is in part true, but they all seem to be peaking later as well. People used to say 24 was the peak year for tennis players, now it doesn't really seem to be the case, lots of older first time champions Wawrinka an obvious one, also Cilic at 26 or whatever and I think Murray was 25-26 as well when he won the US open. Certainly people like Nishikori are looking far better now than when they were 24.
3. And I think an unhappy truth for some is performance enhancing drugs. We have seen with recreational drugs how easy it seems to be for a chemist to tweak the chemicals of a drug to make it into something different which because it's new has not been banned yet (most of you will know about the Mephedrone stuff that was big recently, and there are countless others). And for anyone to think that nobody is doing this is naive, I would go a step further and say I would not be surprised one jolt if many of the top players are employing people specifically for this purpose, with so much on the line it would be very surprising to me if nobody was doing this. Not to mention masking agents which are developed all the time to cover up the use of stuff that has been banned.
Think all three have contributed, most likely but we'll never really know for sure but it sure is interesting to speculate.