Lot of lost opportunity for future Iron Fist stories tbh. Imagine him appearing in Old Man Logan, maybe he can be a wandering hermit in those woods Logan takes the kids through, maybe he escorts them while Logan holds off the armies.
Considering the healing properties of chi, my head canon says Danny can look 18 years old if he chose to. He'd just need to spend a lot of time meditating and channeling that chi. And in longer fights as he used up his reserves I imagine he'd gradually age. Just like the gods aged when they lost the fruits of immortality in the Chaos King thing.
You can make an excuse that his youthful looks are pure vanity, and actually wastes his chi pool somewhat, which would be a neat character flaw that allies can throw in his face.