The adult clothes are for the most part really boring (loove and use the ones with the tattoos though!!) and mostly suitable for mainstream adults in their fifties or something, but I like the teen clothes and would like my adults to be able to wear them. Has somebody modified teen clothing for adults, and where can I find them? I tried to search but didn't find much really... just some gym clothes.

Here are some links, all for females:
http://www.modthesims.info/download.php?t=278939
http://www.modthesims.info/download.php?t=278797
http://www.modthesims.info/download.php?t=276182
http://www.modthesims.info/download.php?t=265017

Thank you arathea! The goth ones are exactly what I'm searching for. If somebody knows any conversions for males I'd be really really happy... the males are the biggest problem, I can't find any cool clothes for guys.

These are not conversions, but maybe you will like this site:
http://www.clubcrimsyn.com/sims2.html

Do a member search for cloudlessnights, I think she has a few teen-to-adult conversions for your guys!