[yarcob]

Having lots of muscle mass isn't healthier than being slim. It's healthier than being fat, but being slim is going to be a lot easier on your heart, joints, and other organs.

(Have a look at people who are 80+ and fit. Most of them are slim. You don't see a lot of bulked up old people)

[zeku]

Your overall point might be correct, but your old people one is just wrong.

Old people aren't super muscular because you need high levels of testosterone to support musculature, and it naturally declines with age and particularly tanks when you are elderly.

[nradov]

The digestive system also gradually loses the ability to absorb protein with age. Elderly people actually need more protein just to maintain muscle mass, but most of them fail to adjust their diets accordingly.