To determine if bipolar itself gets worse, you'd need totally med-free, drug free and booze free bipolars to observe.

Now how many such are out there?

As for bipolar getting worse with age... life itself gets worse once you peek in your twenties. Memory gets worse (for example languages... you are never able to fully learn new language after age of about 12 I heard. You will learn, but never perfectly, they say). Any "normal" who started uni in their 40s could tell you. Life experiences wear you down a bit. Kids are enthusiastic for everything, adolescents somewhat are... adults? Look at any noble cause protest. Very few older people there. Did their tolerance for wars, bad economic decisions and environmental damage get "worse" with age?

I think it has something to do with novelty of things. Seriously, for people in their 50s not many things are so "new" anymore and there maybe previous bad experience.

Imho, enthusiasm has little to do with bipolar and more with life. Yeah, you can get it back, but it's more about... attitude. I have a little excercise I do... "let's pretend you came from far off Galaxy and this is your first day on the planet". Try it. It makes things interesting.