I think generally people start to look older, and I dont mean old, I just mean an age where people stop referring to them as a young woman from about 35, whilst most women seem to feel that they look younger than their age, in my experience I think that a good level of grooming will make a woman look better, but not young, signs of ageing set in properly in mid to late thirties.
and like trim says, its about perspective, a man of 60 may view a woman of 40 as young looking, where as a man of 25 may view the same woman as old