Because humans count BIRTHdays. Plus, conception is harder to pinpoint, exactly. Therefore, counting in years, from the date of birth, is the standard worldwide. (Except in societies or social groups or religious groups where birthdays are not celebrated.) This has been the way, going back as far as 3000 BC, at least, when there is documentation of an Egyptian Pharaoh's date of birth.
MMMFoodies t1_jaet3bs wrote
Reply to ELI5: Why aren't babies 9 months old when they are born? by Best_Bug4471
Because humans count BIRTHdays. Plus, conception is harder to pinpoint, exactly. Therefore, counting in years, from the date of birth, is the standard worldwide. (Except in societies or social groups or religious groups where birthdays are not celebrated.) This has been the way, going back as far as 3000 BC, at least, when there is documentation of an Egyptian Pharaoh's date of birth.