Air layers can work on wood of any age. Actually young wood, first year wood, or the same season's wood might not be strong enough to support the weight of the moss when the air layer is put in place. I definitely would use 3 to 10 year old wood. Find a spot with interesting bends and twists. No sense air layering a boring straight segment. When wood is beyond 10 years of age you might need more time for it to air layer as the tissues may not be as "pluripotent". But in theory there is no age limit, young or old for air layers.
I have never sealed the bottom of an air layer, and have had no trouble with the core of the tree becoming hollow. At least so far. Most of my trees are less than 25 years in my care. I don't see the trunk becoming hollow being a serious issue in the average lifespan of a bonsai. With ume the aged, hollowed out trunks are part of the "magic" of using ume (apricots) for bonsai. As I think about it, it might be worth encouraging hollow trunks to form for future interest as the bonsai ages.