polly - maybe your mum looks really crap in short skirts, but to assume that all other 39 year-olds do bar celebrities is frankly illogical - celebrities do not come from Planet Celebrity, they're just ordinary people.
I bet if you saw me you wouldn't guess I was 39, short skirt or not - but maybe that's because I don't dress like your gran, as you seem to think all 39 year-olds should.
For the record I have a nearlyteenage daughter, who loves my clothes and is more than happy for me to wear short skirts etc. And I would certainly wear a minidress to the soft play, or anywhere else I fancied (well, maybe not in this weather, bit nippy...but in principle, yes).
Do you imagine that 39 year olds just resign themselves to a live of shapeless jeans and fleeces for the school run, or sensible skirts for the office?
Acually, we wear whatever we feel good in, the same as we did when younger.
That's not to say we dress identically to when we were teenagers - I think on the whole, we have a lot better taste, don't feel the desperate need to conform and follow the fashion pack slavishly, in the way teenagers do, and in most cases have more money. We've learnt through trial and error over nearly 30 years of buying our own clothes what looks good on us and what frankly a bit unflattering.
You seem to have this strange idea that wearing short skirts is 'young' or exclusively the province of teenagers. Not clear why - is it because you see short skirts as sexual, and are disturbed by the thought that women old enough to be your mother might be attractive, or have sex? Does Freud come into it in some way?
There are things that I would say are definitely 'young' and that I wouldn't wear at the age of 39 - I wouldn't wear Hello Kitty, say, or over-the-knee socks, or clothes designed to make me look pre-pubescent. Don't think short-skirts fit into that category, really - obviously, you're not old enough to remember the 80s (hardly born then, lol!) - but if you did, you'd know that every woman who wore a skirt suit then combined massive shoulder pads with a v short skirt - eurgh - I remember the look well.
One day, you too will be 39, polly. And maybe you will indeed dress just like your gran.
But if so, that will be your CHOICE - entirely up to you. And maybe your teenage daughters will love it, or maybe they'll plead with you to be a little more hip, please mum!
What your DH, who likes his younger bird, will think about your dress sense - well, let's hope he's joined you in flat cap heaven. :)