An e-bike, such as Bosch electric bike, could be a good choice for a teenager.
Riding an electric bike is a good exercise, in fact, better exercise than a regular bike if you get out more on it, which most e-bikers do. Riding an electric bike is so exhilarating that it’s easy to forget you are exercising, like playing sports.
It all depends on whether its a girl or boy, how old they are, and what type of activities they would do with a bike (off road, mountain biking, speed biking, etc). A mountain/BMX bike is good for those who want to do off road and jumps. Ten speed is good for long rides on the road