I'm 32, I honestly believe that most young women that are in their late teens to early 20s would be considered at least cute or attractive. Women in this age range are in their prime years when it comes to their looks and fertility. There are unattractive women in this age range but they are more of a minority. The one thing that ruins a woman's attractiveness, including those in this age range, is when they become quite overweight. Women with an extra 10 pounds or slightly chubby, especially in this age range, can still be considered cute, but when you start getting into obese BMI territory is when it really takes a dive, this also goes for men.
With that said, many women will have their attractiveness fade relatively quickly. There are exceptions to this but there is a reason why women don't wait until they are 30 to get locked down by a man, as they realize their looks and fertility are declining quickly. It's a different story for men though. If a man takes care of himself (diet, exercise, etc.) he won't see such a sharp decline in his looks. At my age I pretty much look the same with my physique and face as I did when I was 25.
