I am lucky in that I have no trouble staying trim. I am 72, 5'8" and weigh 150lbs and just vary above or below by a couple of pounds no matter how much I eat or how little I exercise.
Staying fit is a different matter. I have hardly ridden the past year due first to an operation on my spine then family health issues.
My slight depression causes me to eat more, and not always healthily, but my weight hasn't risen and I almost wish it had because then I might be more motivated to get fit again.
Trim and fit don't always go hand in hand.