As Clubby has pointed out, there is a build that is still healthy that is right for you at a given period of your life. You cant be the superfit whippet thin person of your youth in your middle age. However its good to stop piling on excess lard. Be realistic in your aims, whats best for you based on recommended body masses for a given age and build. I'm 2 stone heavier than in my twenties purely by middle aged spread but i'm still well within whats healthy for my age and build. My fathers carrying a bit more than he did but he's 70 and its about right for his age and build.