Hey WhiteRhino!
My experience over the past three months diving in Thailand is there are two main types of dive centers (this is an oversimplification, of course). The "zero to hero" or IDC training "Go Pro" factories tend to cater to younger student divers in training as they follow their dream to travel the world, dive and teach as an instructor. These centers are very good for the younger "Go Pro" crowd, but are not really optimal for the "lets have fun and dive as a paying customer" crowd.
Having said that, I have enjoyed many days of diving with a leading IDC training center and had some great dives with much younger divers. In fact, many of the younger in-training divers are really eager to treat you like a "customer" and enjoy practicing their "Go Pro" skills. Some are really amazingly impressive at such a young age, especially the hard working, serious about diving, students and interns. Naturally, there are also some not-so-great 'zero-to-hero-type' divers, but they can be interesting as well.
My experience on comfort and diving is that age is not a factor as much as personality and interpersonal skills. For example, one of my favorite dive buddies is a young 20 something intern who is both fun and a joy to dive with. We did a rippin' 50 drift dive a few weeks ago and had a great time; and she is around 30 years younger (I joke she is my daughter-diver). She did a great job "watching my back" as we ripped along the reef into the deeper abyss. Age is just a number, as they say.
To provide some contrast, there was an "older" late thirties (closer to your age), "Go Pro" wannabe on the boat one day who did not have the required number of dives to start his instructor training before the scheduled date. His plan was to surface during each dive long enough for his dive computer to record another dive, so he would "qualify" for the next "Go Pro" level. Naturally, I quietly did not have much respect for that guy, who was nearly 40 (your age).
My apologies for the cliche, but my experience diving is that "age is just a number" and there are some very nice people to interact with at all ages; and age is not really the main factor for comfort level and professionalism. Then again, I am not really looking to hang out with anyone after most days of diving (go home, relax and sleep!).
Of course, some of my best dive buddies in Thailand are closer to my own age group, 40s and 50s, but I have truly had come great experiences with much younger divers and will continue to have these experiences in the future. That is one of the great things about diving, isn't it? Diving transcends age and other (superficial) social differences, or so it seems to me.
You asked for some advice. My advice, if you are really uncomfortable with younger divers and/or their lifestyle, is to stay away from "Go Pro" dive centers where there are a lot of interns and trainees, and focus on dive centers that cater more to "fun divers" and "customers" (perhaps resort customers), which divers generally are "older" people on vacation or local retired or semi-retired expats.
As I mentioned (hopefully without too much confusion), the business model for the "Go Pro" internship (often zero-to-hero) dive centers is very different than the "fun resort diver customer" DCs; and I find that often in the "Go Pro" centers, the customer is more of a "training object" for the interns to practice their customer service skills (this can be both good and bad, depending on the person and the day of course). It is really quite interesting that the interns are paying to work and learn; and the customers who are not interns are paying to be "customers" so the interns and DM trainees have "customers" to practice on.
It has been many years since I have been on Koh Tao, but I look forward to checking out the diving scene there in the not very distant future. Personally, I like both (over generalized) types of dive centers. I have learned quite a lot from divers of all ages over the past few months, and some of my most cherished memories are with younger divers who I would never have known if it was not for diving together.
You could be Samui based but then you will farther from the dive sites. Some of the youngsters really know there stuff, so it should not be too bad.
