The idea of weight lifting is to improve your ability to lift
weights. If you can not do three groups of 6, you are trying to
lift too much. If you can do three groups of 10, it is time to add
more weight. You do the amount of weight that is right for you. Age
does not matter.
I am 14 6 feet tall and weight 215 pounds. Today I did 10 reps
of 220 pounds, I just started lifting this year and there are 14
year old that do more so...