(see: that last thing I wrote)
Early 20s: 20-23
Mid 20s: 23-26
Late 20s: 26-29
Those years where there's overlap are where distinctions like "early-mid" and "mid-late" come into play; their usage is wholly optional, but I prefer sticking with it to be slightly more accurate while maintaining slight ambiguity.
Obviously I'm making most of this up right at this moment.
Fun Faqt: I was born at 3:33 AM, so I'm pretty much required by birthright to have some amount of 3s as my lucky number.