IMO, it's when women are in their late 40's that their looks start to fade... Many women I know looked 5-8 years younger than their age in their 20's and 30's and even early 40's. Then when they hit 46-48 y.o. they suddenly looked their age. (Me included!)
I don't know a woman over 50 who looks younger than 48-50. Not one. And some look several years older. A few wear make up and dye their hair and so on, and dress 'trendy' and look 7-8 years younger from a distance/at first glance. But when you look properly, they look their age.
As has been said, women who are pretty - large eyes/full lips etc, tend to age a bit better, but even then, they look their age.
As for smokers, they age about seven years earlier.
Having said all this, many women 48+ are still attractive, and look good for different reasons than the youthful cuteness. But you do need more help to look really good when you pass your mid 40's.
I look really tired and weary and OLD with no make up. Like, 5 years older.
Also, I have to add that I don't think women age any worse than men really. I see as many 'rough-looking' men, as I do 'rough-looking' women. But I do think men tend to wear grey hair and wrinkles better. I think grey hair on a woman under 60 looks terrible. (JMHO.)
A few people have said 'what is all this fuss with youthful looks?' and 'I look better now yada yada,' and I am glad they are confident and happy with their looks. But no way is a 50 y.o. going to look as good (physically) as a 25 y.o.
I was gorgeous at 25; slim, fit, and toned, gorgeous glossy wavy blonde hair, pert boobs, nice white teeth, soft silky complexion, great tight bum etc...... but I didn't realise it at the time. I thought I was a bit plain and fat too! (I was 9 stone and 5 ft 4-ish.)
What I would give to look like that now. 
It's true that youth is wasted on the young.