When you hatch your tamagotchi baby, it will turn into a toddler the same day, after a nap, unless you hatched it very late. The next day, it will turn into a teenager. Two-three days after that, it will turn into an adult, and 6-7 days after that, it will turn old if you haven't bred it by then.
Keep in mind that pausing it slows the lifecycle down, and it may evolve later.