I've been watching Warehouse 13 since I was 8 years, and am now 14. WH13 is one of my all time favorite shows!
Warehouse is much lighter and fluffier than a lot of the scifi shows on today, which are getting progressively darker. The two main characters, Pete and Myka, track down supernatural artifacts from around the world to keep humanity safe. There are lots of funny moments, and every single character is amazing.
Pete, Myka, Claudia, and Steve are all great role models for kids and teens. Pete, who occasionally makes rude jokes, is really a sweet guy who is very loyal to his coworkers. Myka is clever and independent, and a wonderful female role model. Claudia really shows young viewers that girls can do anything and go into the science fields. A lot of people seem to have a problem with the fact that Steve is gay, but it's not really a major plot point in any of the episodes, and is rarely touched upon.
As far as profanity, there's really nothing to worry about. Nothing stronger than "damn", "hell", and the like is used.
There are no sex scenes, although sex is implied in a few episodes. Pete and his girlfriend Kelly are shown making out in bed, but nothing really inappropriate.
Violence isn't really an issue. The Warehouse agents use a Tesla, which is the equivalent of a stun gun. Real guns are rarely used. There are no intense injury scenes.
The only thing I would have to say about WH13 is that some scenes might be considered disturbing or scary for young children. In some episodes, main characters are possessed by artifacts and in Season 4 part one, Artie becomes the 'bad guy'.
But all in all, Warehouse 13 has been a huge part of my life for 6 years, and is the first tv show that I ever really got hooked on. I remember anxiously waiting each week to watch the next episode of WH13 with my dad, and I think that this show is inspirational and wonderful.