It is a great era now.
Although, 3 years ago Djokovic was a Murray and could not beat the top 2. 2011 was a miracle year for him or anyone else with that record. He will NOT repeat that. He had some lucky wins 3rd set breakers, the miracl eshot in USO SF against Fed, 2 5-set wins at AO 2012, and everything falling into place including his hot GF. He dated that gorgeous Olympian South American too. Yes, Djoker is a great player no doubt.
But just as you say age catches up with you , shots you get in your mid 20s, you may not fetch in your 30s. So, the legs of a grinder / backboard player have a limited peak.
The example is Nadal. It could happen to Djokovic although he developed a better serve while Nadal's serve has been on the decline. Once the best part of your game incurs nagging injuries you will not be at your best, and lose those close long rally points you used to win.
Since Federer has not really lacked in footspeed in his 30s it bodes well for Murray and Djokovic. Nadal's may just be hereditary.