For me I would choose 21 because at this point you are an adult just by law but truly you are still very young and with lots of energy, fresh brain. So if I can be age 21 forever after year of being 21 you would have made lots of accomplishment because you already have lots of years of experience like this joke of employing a Person who is thirty but has forty years experience 😂. So still having all the benefits of youth since twenty is at the peak of youth.you will be able to make a difference not just to your life but even to the world.