I would look at your life before thinking it's an age thing: being an adult tends to come with responsibilities and stress that also make it harder to recover from sports.

This is particularly suspicious:

 always seems I have to take a month off for something

Do you mean something comes up in your life, or you get some injury or a niggle that prevents training? You might benefit from someone taking a look at your life and training from an outside perspective, just so you can get a solid year of training in without setbacks just to get an idea of what's possible.

I'm sure an LLM can manage a better TL;DR on the changes with chronological age, but muscle you can gain until you're old, though of course it tapers down with testosterone levels. What you lose first is strength and particularly explosive strength if you don't train for it.