Teenagers with cancer don't really fit into a kids group or an adult group. You are too old for a pediatrician never mind a pediatric cancer group full of little kids. You are too young to be treated as an adult because you aren't able to make your own medical decisions or, if you are 18 or 19, all of a sudden HAVE to make your own decisions.
Also, as a teenager, doctors ask embarrassing questions about bodily functions you wish everyone would ignore and then there are all these people who look at your body. When I had my thyroid cancer surgery, I remember waking up in a different johnny than I had gone into surgery with. This meant people had seen me naked! How appalling!
I have a friend who had cancer at a similar age, both of us nearly 30 years ago. Back then no one talked about cancer. Support groups were for old people. We were both, separately treated - I didn't meet her until 15 years later - were told basically to just resume our lives with no support.
Times have changed and its for the good in some ways. But still the teenagers who are dealing with cancer, need more support to meet their needs.