It's a good question. We're mid-fifties and have been together since our mid-twenties. Over our first few years together we tried out pretty much all of the things we fancied doing. Some were fun for a while but probably we'd not be too bothered about going back to these days, or are just impractical. Been there, done that, kind of thing.
And the arrival of a young family (fantastic as that is), increase of work pressure, life in general etc invariably hampers imagination and energy and opportunities to explore anything new.
But that's not to say it can't sometimes happen. And rediscovering something we've not done for years can have a "new" and slightly naughty feel about it
But generally in terms of new/adventurous things that we'd want to try there's probably not that much left, although we're open to ideas, some of which can come up on this forum!
Sometimes a holiday away for just the two of us (we're now just getting to the stage we can do that) can stimulate impulsive and exciting ideas. And new things can occasionally just pop up out of the blue. Just this morning I posted something about our idea of combining pelvic floor (kegel) muscle exercises into sex. Probably not exactly revolutionary or particularly adventurous in the grand scheme of things but an example of new idea for us which we'd like to try out.
I don't think new and adventurous stuff is just for younger people although invariably a new relationship is likely to be a time of exciting, sexual discovery which comes much higher up life's priority pile than in subsequent years. But I'd say that by the fifties you have a much better idea of what you really both enjoy (obviously crucial in a long-term relationship), what is never going to float your boat, but - and I think this is the important thing - keeping an open mind that there may just be something new to suggest or explore.