A tree is a tree is a . . .(until you cut it)

I have just learned about tree rings for perhaps the tenth time in my life, but no one has yet been able to tell me how you work out the age of a living tree. We have two apple trees, stand-alones in a magnificent field before the diggers came in and made a hole for our chalet. They had not been pruned in 20 or more years and when the pruning man came and did his necessary but traumatic work, he ventured that they were 75 and 100 years old.

Could he really have known that?