The main discussion boards for teen and adult LEGO fans are as follows:
EuroBricks Forum - this is one of the oldest and most established LEGO fan forums with a huge number of discussions going on at any given time. You can find everything here from news, to custom building, to contests.
Brickset Forum - this is the second largest LEGO forum, mostly focused on collecting LEGO sets.
Reddit Forum - this is a very active forum in the standard Reddit format, focusing mainly on breaking news, shopping deals, and helping out those who are new to the hobby.
BrickLink Forum - this is another very old forum, mainly focused on buying and selling LEGO.
BrickPicker - for those who are into buying, selling and investing in LEGO. It has set & build reviews, availability information, deals on buying LEGO etc.
There are many other, smaller forums specialized on a specific topic (like customization, castle building, Technic building, LEGO Star Wars, etc.), but if you participate in any of the big ones mentioned above, you will be well connected and informed.