This is really not as big of a problem as we thought it would be.
Since FIREing in 2017 in our early 40s, we find that the age of those we hang out with is much less relevant than we thought it would be. We find that we connect with people of all ages, from stay at home parents in their 20s to active retirees in their 60s and 70s, and all those in between. We are learning more about people and relationships, and now feel that having things in common are far less important than we once thought (just like as a teenager I thought my future spouse should share all my hobbies and interests, and I realized later that this was really not all that important).
When we want to spend time with those our own age, we can do so during times they are available. For example, before FIREing, our Saturdays and vacation time from work were crammed with errands, home repairs and improvements and family activities that couldn't be fit in any other times, so there was little time for friends of any age. Now we get most errands, home repairs, etc, done during the time we would have been working, and our Saturdays, holidays, etc, are now much more open for spending time with whoever we choose.