I can think of a few incidents where this occurred:
When I was 21 and my older sister and went shopping at the local mall. One store was going out of business and I wanted to buy something in it, but they only accepted cash or credit cards, and I didn't have a cc or have enough cash on me, just cheques. My sister piped up and said that she would buy it on her cc and I could write her a cheque. The sales clerk said, "That is so nice of your mom to buy that for you." My sister was mortified-she was only 24 at the time.
When DS2 was born, my SD was 19-old enough to be his mother. We were at the grocery store one day, when DS2 was about 2 months old and still in his infant carrier. SD was playing with him while I wandered several feet away to look at something. AN elderly woman came up to her and said, "It's so nice when babies smile at their mamas." SD just smiled at her and nodded, not bothering to correct her.
My niece was always was assumed to be my child, even when we were in public with her mother, my SIL. (My niece and I looked alike, even our baby pictures were very similar) My niece and I had blonde hair, blue eyes and fair skin, while SIL had brown hair, green eyes, and olive skin. It drove my SIL insane that people didn't think niece was her child. If it was a stranger, I didn't bother to correct them.
DH was always assumed to be his brother's father. DH is 16 years older than his brother (yes they have the same parents).