Your ability to handle weed has nothing to do with age, not to mention smoking before 21 is probably very bad in the long run. Your body gets used to copious amounts of THC if you just keep smoking every day for some time, probably 6 months to a year. Everyone can do it, imho, but nobody even considers it because of the social taboo. Everyone is ok with drinking caffeine till the point you get headaches if you stop, but smoking a joint a day is still seen under negative light.