Have you ever had to reset/download to fix a glitch or replace the batteries at any point? Resetting and downloading resets the age counter, not the evolution counter, which makes your Tamagotchi appear to evolve at a young age.
Sometimes your age counter might just be slower by a couple of hours- I think someone said that their v3 was like that. It's quite normal for a Tamagotchi to evolve to a teenager at age 0 and age a couple of hours later anyway, it's happened to me and some other people before. The age counter isn't always accurate, it just gives you a rough idea of when your Tamagotchi's milestones will be.