Some Tamagotchi evolve to adulthood at 3, others at 4. A majority of mine have evolved at 4. Sunny, mine running now, 32nd generation, evolved into a teenager at 0, so is an adult now at 3. Some other Tamagotchi I've had did not evolve into a teenager at 0 and they evolved into an adult at 3. It only seems to happen 15% of the time.