Scientists have measured 'cool' and determined who has it so teens don't have to worry anymore; they can just ask the adult with the pocket-protector and clip-board.  In a neat piece of social science researchers looked at how kids made themselves popular at age 13 and followed them through to age 23.  Kids who did things to look older at age 13 in order to be more popular, such as engaging in delinquency, hanging out with good-looking peers and engaging in romantic relationships were less socially successful at 23.   (It seems to me there may be a tad of wish-fulfillment of grown-ups who were unpopular 13 year olds).  This is moderately interesting for people who work with kids and for parents of teens. 

The researchers conflated popularity and 'cool'.  Scientists like things that can be measured; popularity among thirteen year-olds is relatively easily measured while cool isn't.  The kids I thought were cool when I was thirteen weren't necessarily engaged in 'pseudo-mature' behaviour and weren't necessarily popular.  James Dean's character in Rebel Without a Cause which the authors adduce is a loner not a collector of pretty people.  Rather the kids who seemed cool to me did whatever they with seeming ease.  The Italian's call this sprezzatura.  Castiglinone in the Book of the Courtier has one of his characters say:

Pseudo-maturity doesn't play into this definition of cool.  Of course it can be exhausting to be attuned to how one appears to others at all times and probably detrimental to one's cool, as well, but that is a feature not a bug of adolescence I suppose.  Now that we have adopted pseudo-youth for adults as an unquestioned value it is probably a feature of middle age as well.   One of the features of Castiglione's work is the way in which people of different ages reflect on the blindness of others and in turn betray their own prejudices.