Perhaps while they are under 30/35. After that it’s really hard to keep muscle mass without doing anything, and esp women need weight bearing exercise to keep their bones strong later in life.
Discussion
I don't know about women. I'm not a women.
For men, It's only hard to keep muscle if you develop your muscles beyond what is normal. I see a lot of guys go crazy with their muscles and then yes they will lose it if they don't keep up on it.
You don't really need extra big muscles other then for show. You can live perfectly fine and stay fit and healthy as long as you don't eat like a pig without even working out.