A lot of people feel that it’s no one else’s business how old they are, and they’re offended by the question. Age is one of the most sensitive topics for many people since ageism continues to be a problem in society.
It’s understandable, although rude, for people to ask at your birthday party. However, it’s unconscionable for someone to randomly inquire about how old you are. Whether they have a reason to ask or they’re just plain nosy, it’s none of their business, and you can decline to respond while still maintaining good manners.