Which workout is best for 13 year old?
It depends on what u need to improve on. Core, muscle, endurance, fitness etc.
Think back 50 to 100 years back 13 year old's had to work what would be considered an adult job now.
There are millions of Boxing, Karate, Judo, Taekwondo, MMA, etc etc all catering for 5 year olds to 80 years its a start to keep fit with cardio and build musle, stay healthy and learn the skill of self defense. I would recommend this 100% millions of children and teena do this around the world with no ill effect.
Cycling is an excellent sport, something people dont do often in this computer age...when I was 9y onward I would leave the house at 7am and ride until I was hungry in summer this could be 6pm something most adults would consider a heavy workout was me playing out.
Its a complete myth that children and teens cant 'workout' schools dont do PE and sport for nothing!