Look at Daniel Radcliffe and Rupert Grint when they just entered Hogwart as first-year pupils, and look at the way they look in the latest Harry Potter movie.
Or, examine the difference between Elijah Wood when he starred in "The Adventures of Huckleberry Finn" or in "The War" in early 1990s, and when he appeared as Frodo in "The Lord of the Rings" trilogy in early 2000s.
Now compare that with the difference between Dakota Fanning, Anna Sophia Robb or Kate Maberly when they just appeared in the big screen and when they were last captured in camera.
Sense any difference?
Well, I guess we all agree that in term of physical and more specifically facial appearance while the boys are -- pardon me – turning arguably rather disappointing or not as handsome as they have been expected to be, the girls are on the contrary turning even prettier.
The actors and actresses I mentioned above are not the only case; I guess you can also see such 'phenomenon' among other celebrities and people around you, and maybe even in you yourself.
And this applies not only on Americans and Europeans; you can see that too on Indians, Indonesians, Arabs, and so on (cmiw).
In Indonesian case my fellow Indonesians can perhaps identify the 'phenomenon' (I don't know what to call this) in local celebrities, such as Sherina Munaf and Derby Romero, Joshua, Agnes Monica, Eno Lerian, etc ... (feel free to fill the blank). While the girls are turning notably pretty or prettier from plain or just agreeable previously, the growth of the boys are rather if not very disappointing; people had expected they would have grown into fine-looking young men instead of deteriorating – once again in term of looks (I'm sorry indeed if I hurt anyone).
My friend tried to explain the phenomenon; she said boys are ducklings and girls are goslings. While boys grow and turn into ugly ducks, girls become beautiful charming swans as they grow up.
Well, after the situation is well defined, a large big question remains: WHY ... ?
Is that because girls turn more careful about their looks and thus spend more time to well care of their bodies and the boys don't (they rather let the weathers and surroundings do whatever the latter want on them) ?
Or because the growth of bones in boys, not like in girls, occur rather irregularly?
Once again, this is not always the case. But I guess I can say that this is most of the case however different your opinion and mine are of what constitutes a beauty.
I am open to any kind of theories and discussions, as well as protests and disagreements.
Cheers 
...!
... Thx for reading, n commenting.
... Yes, the beauty care do, and some hormones, I believe; and, of course, as you said in the last few sentences, the power of thoughts contribute to whether ones will turn into swans or into ducks. You know, I've heard that some basically 'plain' people become pretty or handsome in others' eyes because he or she keeps in their mind that they have good looks. 
it is true, although it is not always the case ... for example: me..!! :-P I'm the exception
who turn into swan as I grow older hahahha ... extremely narcistic