This doesn't surprise me at all. Compared to most other forms of social media, lemmy is pretty old-school in concept. Like the earliest forms of social media (USENET, FidoNET, forums, mailing lists), it's based on discussing topics of interest, not following people of interest. Thus, I subscribe to (and post within) "woodworking", not "Paul Sellers" or "Stumpy Nubs". (I do follow them elsewhere, though.)
In addition, it's been pretty close to 20 years since it was standard procedure to go to teenagers for help figuring out "this computer thing" or "this internet thing". Oh, sure, maybe someone my age can benefit from the knowledge of a teenager when it comes to something like tiktok, but the vast majority of even the over 50s have got all the basics and more figured out.
Taken together with the fact that there are a lot more people over 20 than 10-20, I would have predicted that their numbers would be about the same as for over 60. And that seems to be the case.
I suspect that a better breakdown would be 10-year cohorts starting at age 15 instead of age 10, but that might make population-level comparisons more difficult.
Another way to look at it is that lemmy has more in common with FidoNET than with Facebook or tiktoc. I was using FidoNET in my late teens by dialing into a local BBS before internet became publicly available. I'm 67 now, and have just followed the evolution of "topic discussion" over time.