Both my eldest Grandsons are over 6ft tall, one is 23, the other 16, but both are super slim and each weights in at around 10st.
I’m not worried about them as their Dad was just the same build until he turned 30, yet they all eat massive amounts of food, ad tend to snack on sweets and chocolate between meals.
They are very active boys, the youngest is at agricultural College, which involves quite a bit of heavy manual work, the eldest works for an agency, doing anything from factory production lines to construction work, whatever is available at the time, but also trained in the motor industry on leaving school as an apprentice, all heavy calorie burning work, which probably contributes to the reason of why they remain so slim.