Chase should be early twenty. How old exactly? no idea. I mean it's already a bit messed up because of the children, their age was never mentioned either. Pete seems to be between 2-3 years old.
The author probably avoided giving the ages, because of the issues with Korean ages. If for example Chase was 18/19 at his first encountert with Josh, no problem in the US, but in Korea... . And given that Josh is ~5 years older... .
Chase's parents got together again when they were around 28, first child with 29?
Nathaniel is around 2 years older than the twins. And Chase is 3-4 years younger than the twins.
And even if dominant alphas/omegas seem to age slower, Chris kind of hit on Koi (he had thoughts...), so he doesn't really look that old.
So I think Chase is early twenty.

How old are they anyway?