For a post on what makes tall women desirable, I looked up some data on Miss World contestants (see their pictures here). The chart in the Wikipedia article lets you order the columns, so just click that little gray box on "age" and you can see that there are 5 contestants who are just 17 -- and upon further investigation, Miss New Zealand is still 16 and will only turn 17 in December. If the winner is crowned on Dec. 1, that sounds as if she'd still be 16 if she won. My impression is that countries set their own age limits, although no one younger than 16 or older than 25 shows up (the median age is 21, and the distribution looks pretty symmetrical, maybe a bit more bunched around the younger end).
On the one hand, it might seem in poor taste to send a 17 year-old to an international beauty pageant, but younger girls have a lot to recommend them, mostly in their demeanor, which is more warm and giggly. I had been tutoring girls that age before I started school, but I forced myself to not even start thinking along those lines. But now that I'm surrounded (besieged) by 18 year-old coeds most of the day, none of whom I tutor, I can allow myself to study what makes them so appealing to guys of all ages.
Sure, the skin is much tighter, especially on the thigh, but they tend not to have fully developed breasts and posteriors, which many men are interested in, so I don't think they have a higher net level of physical attractiveness compared to females in their mid or late 20s (30+ is a different matter). But their personalities do tend to differ: researchers of the Big Five personality traits have found that, although changes after 30 are weak, from about 20 to 30 there is a decrease in Extraversion, Neuroticism, and Openness, and an increase in Agreeableness and Conscientiousness. In brief, during your mid-20s, rough, you become more well-behaved and disciplined, and less outgoing, emotionally unstable and open to new things. *
I think Agreeableness and Conscientiousness are more relevant to potential employers; some would say it pays to have a more Agreeable partner, but I prefer someone who's a bit stubborn and mock-confrontational. Higher Openness means that far more silly ideas will fill their heads -- as they did mine too when I was that age, of course -- but that's easy to overlook, since you know it's just a phase they're going through. The committed vegan activist who's over 30, on the other hand, is overwhelmingly likely to be a sourpuss. At the same time, youthful curiosity is pretty attractive. I also prefer someone who's above-average in Neuroticism, since otherwise they won't empathize when I get set off by little things, and since a certain degree of emotional vulnerability makes you more careful about how you treat her.
Higher Extraversion means they're better flirts, are more chatty, show greater warmth, and experience positive emotions more frequently. This is probably the key personality difference that men pick up on when they become entranced by a 20 year-old. Females seem to smile less as they age, and giggle even much less. Now, giggling is one of those things that you can't mention without invoking the slanderous image put out there by non-gigglers that this activity consists of acting retarded and ditzy. But it's not that at all: they're not laughing at something they think is funny, but rather signaling that they enjoy interacting with you, as when a cat or a guinea pig purrs when you pet it the right way. It is just adorable. **
Younger girls are also less guarded and calculating when they're trying to get your attention, largely because their actions are not as under rational control as they are at older ages. Just to provide a few examples: they toss and tousle their hair more conspicuously (fidgeting with hair is always a sign of interest). They put more of a bounce in their step when they walk by, often lightly caressing the table you're sitting at, or gently tapping the chair that's opposite you. If they're leaving a room, they'll caress, tap, or cup the jamb of the door as they leave. And if there happens to be a male acquaintance of theirs nearby, they'll laugh and bounce around as hard as they can while his speech goes in one ear and out the other. All of these involuntary tics are clearly designed to grab your attention, and again the ingenuousness is so refreshing.
As a reminder, others have speculated that these honest signals of lack-of-control serve to convince the observer that the sender can be trusted. After all, if you can't help the way you feel, you aren't apt to leave once someone slightly better strolls by. Giggling, getting butterflies in the stomach, showing anxiety and possessiveness over your boyfriend -- these hallmarks of feminine charm begin to flower around age 15 (at the risk of sounding like a dirty old man), and noticeably wane by age 25, having peaked somewhere in the middle of this range, on average.
To be sure, females in their mid or late 20s have their own charms, especially when they resist the urge to plant their feet, in vain, as the river of time washes them steadily forward. For example, wearing sweat-suits ("track-suits") that look appropriate only on high-school girls, or behaving like a spoiled brat, which would scarcely be cute even were she a teenybopper. I keep trying to think of positive role models in this regard, but the US doesn't supply this urgent demand very well, does it? Fortunately, I happened upon a picture of lovely Giovanna Battaglia, a 27 year-old fashion editor who works for L'Uomo Vogue, and whose work can be seen at her website. *** In the comments section of a fashion blog, which you expect to be plagued by pointless bickering and posturing, everyone at The Sartorialist -- male and female -- agreed that Ms. Battaglia sets a fine example for young women out there.
* See the following: one, two, three.
** Never trust an animal that doesn't purr, like dogs.
*** More pictures and info here.