Threads like this always baffle me. I wouldn't have the first clue what clothes are 'stylish' and TBH I don't even really know what stylish means. Fashionable? (Not that I understand that either) Popular? Nice? (Subjective)
Where do people learn this stuff from? How do you know when something that was previously deemed stylish becomes frumpy?
I think that it is clear from this thread that people don't have a shared view about what frumpy is or what stylish is.
So I will add my view. I don't think it is about being fashionable. The clothes in the shops tend to drive what we wear and so if you shop regularly then by default clothes tend to reflect the current style in some way (much more so in some stores than others)
I don't think I would ever be called frumpy. I don't think that I follow fashion (although as above we are all affected by it as it influences what is in the stores). I am 50 and I would still wear some clothes that wore in my 20s if I had them (hemlines are a bit longer now). I don't have work clothes/weekend clothes. I just have clothes, I wear pretty much the same type of stuff each day, its my style so why wouldn't I.
I think you need to dress to your body shape. Know which colours suit you (I wear 6 colours mostly- 95% of my wardrobe is 6 colours). Buy the best you can afford- have less clothes but better quality. Look closely at the fabric- good fabric wears well.
Don't be afraid to have some bold clothes. I knew when I got dressed this morning that at least 1 stranger would tell me that they loved my dress/and/or ask where it was from. Every single time I have worn it someone has commented on it as it is an amazing dress but you might have to try on 100 dresses to find the 1. I tend to shop on line, try it all on at home in front of my mirror which I know isn't one adapted to flatter (unlike many shops) , often try it a few times, try it on with various shoes, other items of clothing that I already own etc Return what I don't really love. If I buy an item of clothing and I haven't worn it within a weekish I send it back as it clearly isn't right.
Buy good shoes. Buy a good coat. I find it really odd that people will pay £1000 for a mulberry handbag but won't buy decent shoes or a good quality coat.
I personally don't think the cut of the jeans matters- no good wearing skinny jeans if they don't suit you (disclaimer I probably haven't worn jeans for at least 4 years). Or that the length of the boot determine sit you are frumpy of not - you need to think about ankle , calf size and calf length when buying boots (exception is thigh length boots- only pantomime boys look good in those) . I disagree with the comment about pumps being frumpy- I think the right flats for your foot shape/width can be very stylish (patent are great as keep looking good).
Simple jewellery, avoid clothes that are cluttered or too fussy.
Good underwear that fits- start each day by adjusting your bra straps.