Here's the thing.

An average 30 year old who is not doing sports will not lose muscle mass by itself.

An average 50 year old will lose 1% per year or so.

If you get older you have to work just to keep the status quo.

>An average 30 year old who is not doing sports will not lose muscle mass by itself.

>An average 50 year old will lose 1% per year or so.

Sure, but the average is what it is because most people barely exercise, especially when they're older. If everyone lived a much more active life with more strenuous exercise, the average would look completely different.

Actually, if you put them all on an effective weightlifting and diet program, they'd still decline. More slowly to be sure, but no amount of exercise and diet will eliminate the effects of age. Same is true of mental exercise. Animals, and mammals in particular, lack the cellular and metabolic level repair mechanisms that would make it possible for us to keep running at peak ability indefinitely. We have no organ level repair mechanisms at all, outside of, arguably, our livers. Time is coming for you.

>Actually, if you put them all on an effective weightlifting and diet program, they'd still decline

The point is the baseline will increase. Someone who strength trains seriously could easily be at least twice as strong at age 40 as somebody without any training. Even if they unavoidably lose 50% of their strength over the next 20 years, that just means at age 60 they'll have the same strength level as a 40 year old who never strength trained (and twice the strength of a 60 year old who never trained, assuming it decays linearly).

> Animals, and mammals in particular, lack the cellular and metabolic level repair mechanisms that would make it possible for us to keep running at peak ability indefinitely.

Given some of the research on anti-aging, it's not clear that this is the case.

The question isn't 'will they decline' it's will they be better than they would otherwise be, and that is unequivocally yes.