@EdmontinaDonsAutumnalHues is right about shopping at places like Matches or Net if your budget allows. Find a designer you like, find an aesthetic, and you'll always be able to find clothes you like, even if you have to stalk pieces in the sale or hunt for dupes.
Someone mentioned cowboy boots, Isabel Marant always has a variation available with every collection, it's very much her brand, so if that's your thing, look what she's doing this season.
That's the thing about 'grown up' fashion, find your look and run with it. Even if you don't have a huge budget, get on Matches or Net, look through all the designers, and find an overall look that speaks to you.
I have a slightly confused minimalist/70s glam thing, both of those looks really appeal and speak to me, so that's how I dress, there might be a bit of juxtapose going on, but essentially the foundation of both is interesting shapes that suit my body, life and particular mood.
My go-tos would be, upper-end high street Joseph, Me+Em, Maxmara Weekend, Raey and a bit of APC.
Designers, Chloè, Proenza Schouler, Jil Sander, MaxMara, The Row.
A few other bits from all-over if I see something that catches my eye.
Re 'age-appropriate' dressing, I didn't wear booty-shorts in my 20s, I've no desire to wear them now, I'm still quite comfortable in an 'arse-skimming' mini though, because I always did, my advancing years haven't diminished my 'look', all they have done is allow me greater funds to indulge it!