In my mid 30s, I've been in Manchester for over a decade and I'm not at all tired of it yet, but I can relate to how you're feeling.

Possibly projecting here, but in my experience the feeling comes from the frequency of visits to these cities dropping off Vs the things you'd want to do there increasing inversely. I get a bit exhausted going to London these days because I inevitably try to see too many old friends in too many different places. Or I do what I visit for and then try to "make the most of it" and try and visit somewhere else on top.

When I was younger I went and did what I was visiting for, and had a great time—there wasn't this self imposed pressure to pack stuff into the visit.

Might be completely off the mark, but that's my self reflection anyway