In Tolkien’s books, young or youngish ents are mentioned. Where I live, though, the ents are obviously a later stage in the life of trees. You only find them in a mature forest that is not well manicured. An ent-woods is likely to attract large raptors and owls.
Ents are distinguishable from ordinary trees first by their lack of branches — one cannot walk around the forest if one has thirty-foot-long branches intertwined with one’s neighbors — and second by their bad skin. The best identifying feature, though, is their hands. During the day, ents sleep with folded hands.
In my neighborhood forest, all the ents are Willows. I keep hoping for a beech ent, but the beech trees tend to fall down before they reach the age to become ents.