I'm 39 too and yes. I know people think I'm much younger than I am. Maybe they don't have my age in mind, but when it's revealed, they are often taken aback. I wear alice bands, lipgloss, jeans, trainers, sweatshirts and bright waterproof jackets. My style is that of a conservatively dressed student (as it has been since I was a student). I just don't appear like the other Mums in the playground who are approaching 40. Most of them are 'womany'. I'm worried about getting older, because I will still have this air of youthfulness, but a wrinkly face and neck. I'm sure people will still think I'm younger than I am, but haven't aged well, i.e. I'll be a 50 yr old who looks 50, but they'll think I'm 35, but look 50.
I might have been an acquaintance of someone for a while, then the matter of age will come up. It might be in a round about way. E.g. A Gran was speaking about a little boy's Mum and mentioned that she thought she was a bit older than the other Mums. I said that I got the impression that she was a bit younger than me, about 36 maybe. She stared at me, kind of incredulously, like how can she be younger than you, if she's about that age? I expect she thought the Mum was about that age anyway, but didn't realise I was in fact one of the even older Mums.
At work, I don't think I was taken seriously, by my boss. I was one of the more mature staff members, but she seemed to speak to me like I was one of the teenage new starts. She knew my age, but I think it was hard for her to attach that number to me somehow.
It's one thing to look good for your age, but another to be perceived as much younger than you are.
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"We act as though comfort and luxury were the chief requirements of life, when all we need to make us really happy is something to be enthusiatic about." Charles Kingsley