While re-directing is, of course, the polite thing to do, I'll just mention that I have an early adolescent who will happily play with much younger children, as well as age-mates and older children. Any time we go to a park or children's museum, our oldest collects an entourage of (younger) children, which is not at all distressing.
Some older kids enjoy younger children. I think you can often tell if they are okay with a younger child hanging about with them by their response to overtures, using the usual signals that someone is willing to be friendly. E.g., Negative: walking away, avoiding eye contact, shutting them out of a group, terse comment, followed by immediately turning back to the age-peer group. Positive: sustained or frequent eye contact, maintaining or initiating conversations, getting down to their eye level, opening the group stance to include them.
I'd hesitate to teach kids that they can't join an established group, or worse, that if they are part of an established group, they can't welcome new members. I think another approach would be to let him attempt to play with them, but preface it by saying that sometimes people have already made plans, or that it might be something like a big kid playdate, where they invited particular people, so of course we can't invite ourselves. So if they don't invite you to play when you ask if you may play with them, that's probably what it is.
My oldest has not had true peers outside of our extended family through most of the school-age years (so far), but is unusually comfortable with a wide range of ages, from much younger to much older (when we visit nursing care facilities, this is the child who has no difficulty sitting with slightly demented (in the medical sense) residents and chatting with them). So I grant that most older kids are not like this.
