@DollyTots
She seriously said 12. She was the owner of the clothes shop I was browsing. I normally get 18/19 but 12 was a new one on me too. It was only after the fact and purchasing something, I realised I was actually pissed off
Well she was obviously taking the piss, because NO person 2 years off their 30's looks TWELVE. That is ridiculous. She was probably just having a laff. As someone said earlier, when someone looks young, like a teacher or policeman, people say 'jeeez, I must be getting old, that cop looks about 10 years old!' It's just a joke, and they obviously don't.
Don't know many people who are nearly 30 who look 18 either. They may do at first glance, but on close inspection, you will look like someone in your mid to late 20's.
I have seen women of 40-ish before, who look quite trendy and young (like 28-30,) and then when you see them close up, you can see their age. Most people look their age, and no-one looks more than 8-10 years younger. Not when you look properly. Why would they? 
I am also very baffled as to why this woman who you don't know, even knew your age, and why she would comment on it anyway. I mean, did you walk in the store with your 2 y.o. toddler, and say 'Hi I am Dolly, I am 28.' And then did the woman answer back with 'Gosh you look MUCH younger, I would have said you were no more than 12.'
Can't get my head round any of it, and am struggling to believe it happened tbh.