Makes sense.

At 25, you'll have accumulated uni/college friends, work friends, still have some school friends and beyond.

After that, you focus more on work and many are in more serious relationships, both resulting in less social time. Plus, you gradually drop and lose contact with uni/college friends each year. And most people seem to only have a few close school mates. It's basically a decade since you left, after all.

C'est la vie. It's quality not quantity though, as they say. You don't need 50 sort-of-friends as an adult.