The evolution stages of the V2 are:
Baby = 1hr, then evolves into child
child = 24 hrs (1 x full day & night), then evolves into teen
teen = 72hrs (3 x full days & nights), then evolves into adult
adult=after 24hrs can have a baby (through connecting with another tama)
These evolutions are based on a tama that has not been paused or had the time changed on its clock.
As with most of the newer versions (V1 - V4.5) it is not so much what age the tama is showing on its stats screen, it's more about how long it has been a child/teen, etc. This is why some ppl say their tama evolved when it was 3 and others say it was 4 - ages vary and are not a guarantee for the evolution. The amount of time (hours) the tama has lived as a child (for example) is more important.