I'm not sure I understand what you mean by tired adults. My point is, that mushroom-hunting brings food. I believe they regain health on a timed basis, as long as their well-fed. As you say, I can't prove it.

Also, I'm relatively sure that there is a constant increase, and a constant decrease to a villager's health that balance each other out. As they age, it would seem to tip the balance in the favor of decreasing health, but the youngsters seem to regain health quite easily. The obvious things to tie increasing health to would be age, sickness, and food. (Have you ever seen "This villager felt better after eating"?)

I do not believe working your villagers hurts them or weakens them, or prevents recuperation. But for the purpose of making them healthier, I believe [spoiler]Cactus-Cactus-Lotus[/spoiler] is a nice potion to have around. I don't know of any other method except patience.