Teenager refers to the age group 13 to 19 because all those ages end with the word 'teen' as in fourteen, sixteen, etc.
Teenagers often begin to see changes in themselves and their understanding of life that makes them wonder about what their parents are teaching them, if they are not in agreement then they 'rebel', but this may just be their way of showing disagreement since their diplomatic skills are not very high.
Teenage life style? That depends on the individual, but most teens like to do things together and have fun.
I read something some place that suggested that part of the reason teens seem so unreasonable is they are fighting within themselves. They must break away from their families and the security they find there yet a part of them doesn't really want to. It is necessary for this break to happen and so this inward battle has outward signs.
Also teenagers have a deep believe that they are grown up and they are adults and nobody can tell them what to do. They see the injustice in the world and they need to speak out so sometimes what we may consider rebellion is a just strong willing to do what they think is right.