Age isn't an issue as far as getting in is concerned, but it will be once you're on a course. I was 46 when I started at the academy, there was a 43 year-old in my intake, a 51 year-old in the course ahead of me and a 41 year-old in the course behind. None of us is with Airservices any longer. The pace, once you get into the sims, is just too intense and we old farts couldn't keep up. We all thought we had what it takes too.
That's not to say someone in their 30s wouldn't get through, but I don't know of anyone in their 40s that's been successful. Those in this thread that work at the academy will correct me if I'm wrong.