Daisy's given a great explanation.
You are not the youngest there by any means. We've just had a 16 year old join.
There are people of all ages at these groups, all the way up to people in their 70's (and probably older in some places!)
There is even a 'Young Slimmer of the Year' award which is open to those under the ages of 21 (if I remember correctly!) Young people are always welcome to join
I've been at SW for 6 years as part of the social team, and I've seen plenty of young people join through those years. They are always welcomed by the rest of the group and, as a whole, we try to make them feel included as much as possible.
The thing you've got to remember about these groups is that everyone is there for the same reason. To be healthier, lose weight, and try to stay on track as much as possible.