I havent' seen S8 yet (come on, Netfiix!), so I just have the first seven to go by, but based on their behavior, I would definitely say they aren't in the teen years yet. And you can't judge it be changes in interests. That happens even at sub-teen years. In fact, it happens quite rapidly sometimes.. I got four nephews who seemed to changed their interests almost on a monthly basis; soccer one month, karate the next... If I had to guess, I'd say they were around 10.
Another thing to consider is the lifespan of each group of ponies. Celestia is over a thousand years old, though this may be an alicorn thing. Unicorns and Pegasi are unknown, but since they are considered "magical" creatures, it's probably longer than Earth ponies. Speaking of which, Earth ponies (equiv. to our ponies) live an average of 25-30 years. Course, we don't know how long a year is on their world.