Here's a little known secret: in most cases, inactivity is a choice!
You don't need to stop at a certain age. So find a starting point for your current age and fitness level and start reaping the benefits!
If you compare where you are now with where you'd like to be, it can seem overwhelming. So start small. Make small gains and protect them. Then progress by adding another small change.
Are you too lazy to feed yourself, turn on a computer or mobile phone, visit the bathroom, open a present, make an excuse? Then you're probably over motivated and soon be coaching others.
Don't be put off by age!