POD. My mother is 44, and a grandmother of 2. People sometimes assume she is their mother (although alot of the time i think people are being polite, just incase she IS their mother, and they don't want to offend her) , so it does work both ways.
The second is a faux pas -- almost certainly not meant to be a slight. The fact is, at 45, you could easily have a grandchild -- so it's not that much of a stretch. Best way to handle this one is to cheerfully (not defensively) say, "No -- she's my daughter." Believe me, the offender will be plenty embarrassed and will probably be cured of making that particular assumption in the future.
Truly, I would much rather the assumption be that I was a child's mother rather than grandmother, and I am
old enough to be a grandmother (I'm not a grandmother, but my daughter is 20!).
On election day 2006, when I was 40 and my son was almost 5, I brought him to the polling place with me so I could vote. The poll worker smiled at him and said, "Are you helping Gramma today?"
GAHHHHHHH. I hastened to let her know I'm his MOTHER.
My sister is 44. She just had her second baby two months ago (her first is three years old).