My husband says all the ways not to act old can’t be depressing (i.e., things he himself does), so as #1 I offer up using language that has no right to come out of lips that are more than 45 years old….make that 35….make that 14.
Using too-young slang is akin to wearing a yellow miniskirt or driving a Zipcar: It makes you seem older because you’re trying so hard to be comfortable with something that was obviously minted by and for a generation that came way after yours. Yeah, I have one post-40 friend who can say awesome in what sounds like her native tongue, but most people in the middle ages shouldn’t try to say anything more modern than “cool.”
Of course, you don’t want to swing too far the other way and use outmoded words like keen, neat, or smart.
Groovy is acceptable when used with irony. Great is always okay.
But rad? Ill? Tight? Please.