There is a thriving beetle-rearing hobby in Japan, mostly centered on stags and Dynastines. Since I'm in the US (where we can't legally have non-native plant feeding insects), I've never had L. cervus. I have kept the very simmilar US stag L. elaphus, however. I've only had males, however, and haven't yet had the opprotunity to try to rear them. They are impressive beetles, they'll toss any other beetles aside, especially if there's food around.
I feed them watered down maple syrup from a dish with some wadded up paper towels in it. IME, wild caught adults live a couple of months.
Yes, like many insects, most of their life is spent in the larval form, and they die fairly soon after reproducing. If anything, I suspect they live longer in captivity. I'm not sure what the developmental period is for stags, but I raise eastern Hercules beetles (Dynastes tityus) and they spend 2-3 years as grubs before pupating and finally becoming beetles. The adults only live a couple of months.