Nothing wrong with looking like yourself as you would at any age, given your particular biology and your environment and the interaction thereof! That's my take on it anyway.
Loved Rae's comment on that, and I want Suz's as a T-shirt slogan to hang on my wall.
That said, the real problem isn't that there's anything wrong with someone guessing our age or with anyone looking like they *really* earned all their 40 years of experience in the big bad world. The real problem is that people make *value judgements* based on age, and often the guessing game is just the start of it. There are people whose sole snide point in telling you that you look 'well preserved' is that you obviously are too interested in looks. There are people who will dismiss your ability at work and compare you unfavourably to X or Y, because you look or sound 'younger' according to them, or you are chronologically younger. And I'm sure it plays out the other way too --- teens aren't the only ones being derisive of the middle-aged and beyond. VC's comment really has something important to say there!
Personally, I've struggled against the age bias at both ends. I apparently have a body that grows/ages in odd spurts, so that I very rarely 'look my age'. At 6, I was repeatedly denied admission by schools because heads and their secretaries insisted I had to be 10, at least (I looked like I'd hit puberty or was barely prepubescent; understandable mistake, but unreasonable in the face of the evidence of a birth certificate). At 10, there were distant cousins who'd decide I was of marriageable age and come up with lists of prospective grooms, or worse, actual proposals --- awfully awkward! At 18, i could pass for 28 --- often very useful, sometimes quite unpleasant in the face of a certain sort of 'persistence'. At 28, I was underpaid for the job I was doing and the position I held --- because no one my age had ever had that position, so I should be grateful I got even that! By 30, family members on each side were pointing out that I look so much older and more haggard than my SO (he's a year younger; we were classmates at university), and some were going so far as to suggest there had to be something wrong with a woman who tried to trap a younger man and with a man who insisted on tying himself to an older man when he could easily 'do better'! (I'll shamefacedly admit here that I tried, for a very brief time, to 'dress younger' to avoid the stares and remarks of strangers at least --- but thankfully better sense prevailed before long!)
Slightly OT, I find my weight does strange and wonderful things to my apparent age. When I have extra pounds on me, it smoothes out my face and renders it younger up close... but from a distance, my body looks lumpier and saggier, hence older! Make-up sometimes has a similarly wrinkle- and dark-circle enhancing effect, so I'm guessing inexpert application is 'ageing' too? It's the cosmetic equivalent of stumpifying clothes perhaps?
Which brings me to the other area of social perceptions I struggle with: height. There are actual studies to show that the taller get paid more (how often have you met a short CEO?) --- a good book on this that addresses the scientific mechanism of judgement and its basis, rather than engaging in a harangue, is 'Blink'. Highly recommended to anyone interested in first impressions and appearance judgement!
Question to ponder for another day: Is trying to dress taller as wrongheaded and counterproductive and stereotype-perpetuating as trying to look younger?