For teens under 18 how much sugar should they have per day and what's the max amt?

None. There is no ammount of table sugar a patient should have per day. Table sugar, brown sugar, etc. Is a sweetener and as such a treat, not a necessary nutrient.
Minimal sugar. There is no quantitative answer for this but the less the better. The glycemic index is more important than the food label. Glycemic index is how quickly something breaks down into sugar. The easier it does the worse it is for you. Eat whole grains, less processed food, avoid caffeinated drinks and soda, eat more plants and lean protein.