I also own a HP t-shirt, but most of my friends don't know about it!
I don't think HP is aimed at children or adults; it's aimed at anyone who wants to read it. The kids wouldn't be able to understand some of the language or any of the hints etc, but they get the main jist of it. The more mature people will understand that it's also about friendship, family and (I hate to say it cos it sounds really soppy!) love. I think the older you are, the more enjoyable the series is, you understand and relate to the characters' feelings.
Sorry to go on...
Danz_angel x