What do older guys do to keep their legs firm?

I'm 65 and noticed as I've gotten older that regardless of what I do - walk, ski, row - my thighs aren't firm anymore. I hate to use the term cottage cheese but that's the only term I can think if that might describe what's going on. I mean, not as bad as my 86 year old mom but you get the idea.

What can I do that can at least reduce that?