1) We know that identifying as an Adult isn't just reaching a certain age. It's a culmination of individual experiences that build upon themselves over time. Yet our society still holds to outward indicators, societal markers, milestones that indicate someone is an adult.
2) And would those measures be accurate in other countries?
I'll give you an example, Here in the United States we've established the age of 21 as the legal adult age to purchase and drink Alcoholic Beverages.