Ok, I am a teenager and I will admit I have cringed at pretty wrinkled ladies wearing skimpy dresses in public, but really I think it should be more about what is flattering on each individual and what really complements their beauty instead of blindly just following trends because they are 'trends' or being..excessive as if that is the only way that they look attractive.
It's not that style/clothing is 'limited' for various ages, but what fits and looks best on someone. If a young girl dresses 'conservatively', people might relate that to a mature woman(for EXAMPLE) and therefore the girl is then thought of as 'trying too hard', because of all the images that have been associated with her style of dressing. Another instance is when someone dresses 'inappropriately', but there is no definite 'borderline' as to what is tasteful and what is tacky, because, again, everything depends on each individual and the fit of clothing will be unique for everyone. A low-cut shirt on a skinny model WILL look different compared to a curvy woman wearing the same outfit. And it is not only their style, but their attitude, posture, the way they walk, how their hair is done, and every other little detail that counts to the result of the look.
So I'd say there is no particular borderline for what is appropriate and inappropriate in the way a woman dresses, and even then it is entirely based on different opinions..but I'm sure women have enough logic to see themselves and decide whether it is tasteful to them, and more importantly, if it fits them and they have confidence about it. Though there are many girls that I am just pained to look at because of their over-exposed skin. A little skin can be every bit as fashionable and sexy, actually, MUCH more sexy, than just dressing in the tightest, smallest, skimpiest outfits out there. Mainly for me I just separate appropriate and inappropriate from how they look in the outfit. If the outfit is so tacky it makes me shake my head...well then instinctively there must be something wrong here, because clearly it must have been an unflattering outfit. Anything that brings 'sl*t' or 'wh*re' into mind is NOT good. But I guess for Christians the 'borderline' is also different, because I have christian friends who disapprove of me wearing a tank on a hot summer's day..