I read an interview about a year ago about the topic of starting healthy until old age. The main points were:

1. You start to lose muscle mass at 30 years and for the average person you lose about 1% per year. In the article, they said that this would mean 50% of muscle lost when you hit 80 so it must've been on average (otherwise you'd lose about 40% total).
2. If you need to make a choice between endurance and strength exercise because of whatever constraints, you should go with strength. Ideally, you do both. But strength is ultimately more important as it keeps your metabolism working long term and also prevents accidents. After all, it's what keeps you skeleton in place.