What happens to muscle as you age?

For you older monkeys who worked out a lot in their youth - how does (muscle) weight translate as you get older? In your 30s/40s do you start to get flabby in the chest, arms, and legs even if you keep working out?

For reference I'm about 6'2 215 right now and wondering if I should start slimming down now rather than later - pretty high on the BMI scale for someone my height, although its not all bad weight since my abs are pretty visible even when I'm resting