While I was on the phone earlier and before I’d put the pumpkin out, we had our first trick-or-treat knock on the door. I couldn’t end my phone call as it was the police phoning (I was assaulted a few weeks ago). My 10yo son asked if he should answer the door and I gestured “no”, but he wilfully misinterpreted that to mean yes and did so anyway. I couldn’t see the people but could hear them. DS gave sweets to the little girl then I heard the woman ask “and how old are you?” DS replies “um, 10”, and she said “oh, a big boy”.
I was instinctively 
that she’d asked him this. Her child was apparently about 4 years old, and DS doesn’t know her so it’s not like there was a reason to ask. Maybe she thought he was home alone and was judgey? As I was in the next room I’d assume she’d heard me on the phone. The Q does ring particular alarm bells for me because of an unpleasant incident years ago when I was about 13; a man stopped me in the street by asking me the time, then my age, and then tried to persuade me to get into his pickup truck to come and meet his daughter.
But that aside, DS was uncomfortable at the Q which is the reason why I would never ask a child who was ostensibly on their own their age or any other personal questions. I’m sure the woman didn’t mean any harm but AIBU to think it was inappropriate of her to ask?