@rowan phil,
I thought this would be about 20-29 year-olds who act like teenagers. That's something I can relate to.
Like, the naming concept for -teens is basically what you would call a multiple of ten for that number with -een instead of -y. Except for 3 (thir) and 5 (fif), 4/6/7/8/9 are all just with a suffix added.
So basically 20=tenteen, 21=eleventeen, 22=twelveteen. This makes a full 10-year span. It may need to end at 23 though, because it'd have to be thirteenteen and that's just ugly. Unless you cant to call it Bakerteen. I suppose 24 could be 'Tweekteen'.