Dave,
You don't need to accept being old, you can fight it. I just had my 51st, went to the gym and pumped some iron to celebrate. There are older guys than me there and they're in great shape, I'm a novice in comparison.
Muscle mass goes when you stop using it. I pump iron three times a week and do some sort of exercise on the off days, walking at a good pace for 30-60 minutes can be a great exercise. But key is maintaining muscle mass and for that you need to do resistance training of some sort: I choose to lift weights. Just do it.