Deep vein thrombosis?

How common is it in teens/children?

I would say it is not very common... but look at the list of risk factors to see if they pertain to you... a DVT can happen at any age.

Risk factors for developing a DVT are the following:

1. Having a recent surgery or recent injury

2. Immobile / Inactive (this refers to people who may be bedbound due to age or illness; or people who do not move around due to a prolonged hospital stay or paralysis)

3. Having a Pacemaker

4. Blood clotting disorder

5. Sitting for extended amount of time such as flying or driving long distances

6. Pregnancy or being on Birth Control Pills

7. Smoking

8. Obesity

9. Having had a DVT in the past

10. Chronic illness such as Heart Failure or Cancer

I read these statistics on it:
7% in children
10% in adolescence
http://www.surgeongeneral.gov/topics/dee...

Fully considering the prevelance of video games....I would say that DVT stats for children are going to rise. Just recently a teenager in Japan died from DVT after playing World of Warcraft for several hours sitting in his computer chair. Kids need to be informed that getting up and moving around every half hour is a very good idea.

It's not common, but some things which increase the risk are lack of movement (from sitting on a plane or bus, or being bed ridden) for extended periods, or having had recent surgery or an injury.
Symptoms are pain, redness, heat and possible swelling in the area. Lower legs are a common site, but any limb can be involved.
If you are concerned about this, then see a doctor. An ultrasound scan can be used to confirm the presence of a DVT.
Preventative measures include encouraging movement. Everything from walking around to wiggling feet and toes is beneficial. If movement is restricted (for whatever reason), then anti-embolic stockings are often recommended.