twenty-something
twenty-something (or twentysomething) is a term used for people at the age of 20 to 29. its is used instead of "twentyfive years old" for example, because at this age its harder to tell how old someone is (by looking at him/her) than it is in the teenage years (and  often it "matters" less).
dude #1: how old is Tim?
dude #2: I don't know.. he is a twenty-something, thats for sure
dude #1: thank you for this exact piece of information