I'm in my late 30s now and this question has really been running through my head - I have more money (not that much but enough to let me buy some things from therealreal) and more confidence than I did in my twenties, and I've enjoyed using some of it to buy things I like and try out things for the last few years. The main change so far was that I can't really get away with being scruffy anymore, like my regular uniform of a band tee and cutoff shorts had to be kept for home instead of being my outfit to the pub, for the last few years I can't really get away with cute little sundresses that expose a lot of shoulder or chest anymore, etc. Now 40 is not that far away and I'm wondering what change will come after that, because I don't look like most people's idea of late 30s, probably younger than that. But neither do I want to dress like some stodgy image of a "mature" person, 35 year olds don't have to be trussed into a pencil skirt and 'tasteful' navy the minute they hit their birthday. I mean, Debbie Harry only got famous in her thirties and at 35, was considered one of the sexiest women of her era.
I was in many ways an indie girl in my late teens/early to mid twenties, and some parts of that stay with me even now, like my fringe, a love of graphic tees and layering, but so far, as I got older I'll make sure I put a good jacket over the band tee so only a bit of the shirt shows, that if I'm wearing a tshirt on it's own it's not tight, and that if I'm wearing a short skirt (higher than knee length), I must wear it with tights, if I want to wear polka dots then make sure the dress has a flattering cut and a neckline that isn't too low. If the dress is short, then make sure the cut is a simple grownup and flattering one - no cupcake princess dresses, but a knee-length subtle fit and flare A line is good - and it's worn with dark tights, if I want to wear a tight leopard print top then make sure it has a high neck and sleeves. No super high heels now, I've lost the physical capacity to deal with them since I never liked wearing them outside of the occasional party. I have no interest in trying to wear things a teen or 20something will be interested in now. But over the pandemic, my hair started turning grey and I decided to let it grow, and now it's making its presence felt and I know I REALLY have to think about how it looks and how it influences my overall look.
(and I will say, it's also about knowing your tastes and vibe. Debbie Harry I mentioned above, but some women just always carry a certain girlishness about them well into their 30s, 40s and even 50s - see: Lily Allen, who's now 37, or Winona Ryder, now 51. I think carrying that through different ages can work beautifully, there's a certain magic to women who change what they wear, but change it very little. That said, I think it's natural to feel like you flounder a bit between decades, I certainly felt it when I was younger and at 27, no longer a student and didn't feel natural anymore dressing like one, and I feel like at some point if my body and face change a lot then I might flounder then too)