In the US, frailty leads to falling which is the number one cause of injury for people 65 and over. If that isn't a HUGE reason to start moving towards building some more muscle, I don't know what is!
Here are a few reasons to get moving:
1)It boosts your metabolism: A pound of muscle burns twice as many calories as fat
2)It'll keep you young: resistance training can counteract the (potential) half of a pound of muscle mass lost every year.
3)It's a great way to burn fat: interval exercising requires the heart to pump more blood, quicker than traditional weight lifting alone