It's common.
Friends are friends. We've learned to try DS6 with various kids--just as you're doing--and see if he's happy with them. If so, regardless of age, they become a potential long-term friend. If not, then we move on with a shrug. But he does tend to like kids who are 2-4 years older than he is chronologically.
In fact, one of DS6.5's (and, for that matter, DS3.5's) best buddies is turning 10 this month and is quite GT in his own right (though admittedly with some emotional/behavioral issues that probably make his "real age" a bit lower than he might otherwise be given his GTness).
If your DS likes the older kids better, then I'd say don't fight it. Find older kids for him to befriend. Outside of the strict school setting (and even within it often!) chronological age is generally meaningless for friendships. Many kids--especially HSed kids--couldn't care one whit about age.
One other thought: is your DS an introvert or an extrovert? (And please note that I'm not asking if he's shy--shy and introverted are two different things.) Does he prefer time alone to recharge, even if he likes to be social? Does being social make him tired, even cranky? Or does he get energized by being with people?
Are you an extrovert? Do you crave time with people? Your posts make it sound to me like you might be an extrovert. That's important, too.
If your son is an introvert, he may just prefer to play alone a lot, and that's fine as long as it's fine with him. Ultimately, the big deal with friends is going from 0 to 1. If he's got one playmate that he sees semi-regularly and if he's happy, then he's probably not a kid you need to worry about.
If he has no friends of any age, if he's sad and lonely, or if he's an extrovert who is having trouble connecting with people, then I'd say (in my non-expert opinion) that there may be a problem.
Just be sure that you're not trying (with all the best intentions, of course) to turn an introvert into an extrovert. That would frustrate both of you unnecessarily. Everyone needs some social connection, but some people need significantly less social time than others. If you're an extrovert and he's an introvert, the amount of time he spends alone (especially when he's on a playdate!) may seem excessive to you, and the friendships he has may seem superficial, but it all may be perfectly fine for him. It just comes down to how he feels about it.
If it helps, I'm a reasonably well-adjusted and a highly social person, and I'm an introvert. I mostly preferred to play alone, especially when I was under 7, but I made friends, too. I just had mostly acquaintances and a very few close friends, and that worked for me. Nowadays no one can believe I'm an introvert, I'm so social. But I am. I enjoy being social, but I need my time alone before and after those social contacts just as much as I need air!
If your son is happy, if you're giving him opportunities for forming friendships, and if you and the doctor have ID'd no stumbling block to his making friends (e.g., hitting, evidence of social disorders, autism spectrum indicators, etc.), then he's probably fine.
Try events with some 7-9yos involved and see what happens. That may do the trick. And be sure that he gets time alone before the event if he wants it. ODing on social time can make introverts hide during social events...like playdates!
K-
