If it helps, two couples I know, both in their early 30s went a couple of years ago (to the one in the UK).  One especially is lego mad, and loved it.

Evidently the place is arranged at different levels. So while kids might just enjoy that it's Lego, adults can engage in the hunts, the themes and more.

A [review of some of the rides][1] of the UK one points out that yes, 2/3 of the park is aimed at children, but adults will still enjoy many aspects of it, as will a teenager like yourself.

I guess it comes down to how much you like Lego.  If you're fine being a big kid, like many adults I know, you'll love it.  If you're the self-conscious 'must be cool' type, you might get bored.


  [1]: http://www.tripadvisor.com.au/ShowTopic-g186419-i1323-k3070945-Legoland_for_adults-Windsor_Windsor_and_Maidenhead_Berkshire_England.html