For a while I caught the Meles bug and thought the Next Gen takeover was imminent. Now I think the opposite.
Nadal does look like he's aging, and he's not physically up for hard courts the way he used to be, but he's still as dominant as ever at Roland Garros. He looked lackluster most of the tournament last year and still didn't drop a set. It would take a precipitous decline in his overall level for his run at Roland Garros to come to an end. I think he's got another two or three years, possibly more (and a decent shot at picking up a slam title or two elsewhere in that time; he's still making deep runs everywhere).
Djokovic looks even better. He's gotta be aging the smoothest of any tennis player since, I don't know, Rosewall, when the game was nowhere near as physical as it is today. He's hardly lost a step, let alone faced more severe physical decline like loss of endurance or explosiveness. He doesn't even seem to be on a downward trajectory like Nadal does (who's just at a high enough level, particularly on clay, that it doesn't matter). The only age-related problem he seems to be facing is a general decline in his week-in-week-out consistency, which doesn't really matter for the slams, as he only has to peak for 7 matches spread out over two weeks. Barring a surprise career-ending injury, which of course could happen, though there's no predicting it, I think he's got a good 4 or 5 years left of playing slam-winning tennis, and could very well finish several of those years number one as well (though with the decline in non-slam consistency, someone like Medvedev could sneak in and steal the top ranking away at times).
In raw numbers? Nadal anywhere from 2 to 5 more. Djokovic anywhere from, I don't know, 5 to 8 more.