We often group people's ages into decades. To express this, you say someone is "in his 30's" or "in her 80's".
There are two age groups that are different. For people under about 13 years old, you don't use this expression; you just say that they are:
- babies (0-2)
- toddlers (2-4)
- children (1-11)
- pre-teens (11-13).
The other group that's different is people from 13-19. You say that these people are teenagers or "in their teens".
To be a little more specific about someone's age group, you can say:
She's in her early 40's.
He's in his mid-50's.
They're in their late 20's.