Question

When are trees considered to be in the adult stage of their lifecycle?

Answer 1

When they start producing flowers or fruit

Explanation:

There are 5 main stages of a tree's life cycle: Seed, Sapling, Adult/Mature Tree, Ancient Tree and Snag.

http://texastreeid.tamu.edu/content/howTreesGrow/

A tree is considered to pass into the Adult stage when it starts to produce flowers and/or fruit, and can thus start seed dispersal, ensuring the passing on of the trees genes. The time it takes for trees to reach the Adult stage varies from species to species. For instance, an English Oak may start may start producing acorns at 40 years old, while the Rowan will start early at 15 years old.
https://www.woodlandtrust.org.uk/blog/2017/06/life-cycle-of-a-tree/

Other characteristics of an Adult tree is a rigid trunk and a rougher, more thicker bark.

I hope I helped!