I am 45. I use it to describe people who are older than me. But perhaps I am now long in the tooth myself. I feel older than I should now. Sigh. Don't let these Americans and whippersnappers put you off using this fine expression!

I've heard it a handful of times, most recently in an episode of 'Suits', but never used it myself.

Haha I think I might've used it once in my life. I'm not sure if this idiom has extended to millennials, but it's fair to assume that most people understand it. We don't tend to use it, though. It's a bit antiquated.

I have never used that particular expression to express that someone is old, but I have used it to express that something is old, e.g., "These shoes are a little long in the tooth." (The expression comes from the way that a horse trader evaluates the age of a horse, so it isn't typically applied to people.)

We know the expression, but it's a bit of a Catch-22. We will hear it mostly from the older generation, usually to describe how things have changed from when they were young. Ex: Well, I may be long in the tooth, but when I was in school, kids didn't wear their pants so low and show off their underwear.