For example:
"That man is in his early 20s"
"That woman is in her mid 40s"
"That person is in their late 50s"
How would you say "early x", "late x", and "mid x" in Spanish when referring to age?
Colloquially you can say
That man is in his early 20s / Ese hombre tiene veintipocos (años)
That woman is in her mid 40s / Esa mujer tiene cuarenta y tantos (años)
That person is in their late 50s / Esa persona tiene cincuenta y muchos (años)
As pointed by mdewey in comments, you can also use suffix -y pico (y becomes i sometimes depending on the decade as you can see in my examples above). For me, it's not clear in which range I would apply it but probably early or mid. I think that veintipico can be considered as an equivalent of twenty-something.
Click linked words to access DRAE definition for those terms.
Following the order of the question title, it would be:
or more informal: