How old do you have to be to not be a kid?

By age, you will not be a kid by 12 or 13. By that age people expect kids to mature, and that's when kids go through a phase. Normally people go by age, but rarely by maturity of the way you act.

Until you're doing adult activities, as in paying rent, having a job, complaining about money is your daily habit, and much stress. oh, and you never experience kid activities and your mindset is matured. hm ... i dont know