Not so much style as, well, what granny would say, but valid none the less I feel:
Wear underwear that fits, the number of people that I see who have ruin their outfits with too-tight-pants is crazy. Of bras enough has been said lately, much of it very wise indeed.
Don't forget to do neck, decollete and hands, not just just face, with your good moisturiser, serum, sunblock and whatnot.
Good posture is all. Honestly.
Confidence can (and should) be faked, wherever necessary. Nobody at all knows how you feel inside.
Arrange your face.
Doesn't matter how much time, thought and effort one puts into clothing, acessories, hair, make up and maintenance it can all be spoilt in an instant with a scowl, condescending look or sneer.
Jeans and trousers are not obligatory, if they don't flatter you don't wear them. End of. TBH same applies to any item of clothing.
Style beats fashion every. single. time.
The only exception to this is the girls and women (regardless of age) of a certain height and build for whom 'fashion' is intended. Women of this type should, of course, not feel obliged to follow fashion should they actually have a sense of style.
So. FInd out what you love. Develop your style, keep developing it. When some numpty finally decides that some element what suits and flatters you is 'fashionable' buy, buy, buy. But, quality over quantity, natch.
When walking in an unexpected rain shower / having a crisis of confidence / a seam splits / child pukes all over you in public / other unexpected shit happens, stand up straight, arrange your face and walk at a normal speed.
There is no age at which a particular hair style or skirt is too long or too short.
Following style and fashion rules is for those who lack the confidence to do their own thing. So, I repeat, confidence can (and should) be faked, wherever necessary.