My generalized theory, based mostly on observation, is that women have a significant dropoff typically in their late 40s, based on when the most substantial hormone changes are occurring, whereas men experience a steadier decline in performance, because testosterone decrease is steadier than progesterone/estrogen fluctuations.
I'm 48 and female, and what I've learned so far is that you have to be constantly paying attention to things you need to adjust, like mileage, when it's ok to do hard workouts, etc. I used to do 3 seasons per year in my 30s because I was able to ramp up quickly and train hard for a few months, drop down and rest, then repeat. I switched my overall program to be less peaky, because it takes too long to get in basic shape and I can't handle the same level of work even after I get in basic shape. That has kind of worked, but I'm finding that I sometimes get stale at the wrong times, so obviously I haven't totally figured it out yet.
Staying healthy is the first most important thing, I'd say.
The second most important thing is not being married to the things you've done in the past. It's a pain in the ass figuring out what works for you as you age, because it's highly individual, it keeps changing from year to year, and a lot of times you can't tell something isn't working anymore until you've gotten yourself in a hole - at which point you have to get yourself out of the hole and then figure out which thing(s) put you there in the first place. It can be very frustrating. But that's the game as you age. Adjust, adjust, adjust.