Well... I'm almost 35 years old (w/ wife, son, job, etc.), also I'm 800s master. I've been feeling how my mind is slower than when I was in my 20's. I can't play more than 3 long macro games before feeling tired (then I start to cheese : P).

But I feel my main obstacle to get into GM or high level game play is not my age, it's time. Having a family, a job, other responsibilities and social activities prevent me from having my mind focused on my gameplay.