I've been in Muay Thai now almost 1 year and am turning 32 this year.
Too old? Absolutely not.
The club I go to isn't overly large but I'd say the average age of the people who go is early-mid 20's. There have been times when i have been the oldest in class on a given day, but there are a few regulars in their mid-late 30's, and maybe even one or two who are 40 or over.
I wasn't used to being the "experienced" guy in a setting like that until I joined, and by that I mean some of the younger folks asking me questions about what they should do for a career and about things like moving out of their parents' house. That took me a bit unawares at first but I'm used to it now.
The guy who owns and runs the place is older than me but the other instructors are 10 years or more younger than I am.
I'm sure you'll enjoy it, but depending on the composition of your club, you might "realize" your age a bit at first, especially if you enter into a beginner's class.
One good thing though, I worked out fairly religiously most of my life, so once I got used to the types of workouts they do there, now I'm near the top of the advanced class in terms of strenght and cardio, and my technique has come an extremely long ways to.
Just take the plunge man, then you won't have any regrets. I love it and go about 5 hours a week.
"I do not pretend to know where many ignorant men are sure" - Clarence Darrow