In the United States, this would be considered "too personal" to ask a stranger, or someone you just met. Once someone has become a friend, you might exchange this information. It would usually come up as part of a conversation. For example, if someone says to you "Today is my birthday," it would be natural to ask "Oh, how old are you?" In the US, there is an unpleasant reason why a man might ask a young woman this question. You can read about it in the Wikipedia article on the "age of consent," https://en.wikipedia.org/wiki/Age_of_consent .

To ask a stranger "sorry how old are you" is ok but some may find you suspicious. Don't not ask an elder for there age; If you do they may be highly offended or you might just get the same answer "old enoughto be your mom/dad." or "old enough to be your grandma/grandfather."

Thank you reply.

hahaha, in Asian culture it is customary, but in European culture, it can be offensive.

It all depends on a persons cultural background, but unless it's being asked in a professional setting, it's generally considered "rude" by a lot of people.