A variety of mechanisms may be responsible. The most obvious is that attractiveness is associated with the quality of an individual's genes. We can all spot the difference between the faces of grown men and women, and researchers have identified the reasons why. Boys and girls have similar face shapes. At puberty, hormones act on their faces to masculinise or feminise them and produce distinctive features of mature men and women.
Female faces remain relatively childlike.
High levels of oestrogen in growing girls prevents the growth of facial bone, and leads to increased thickness of lips and fat deposition in the cheek area. Theoretically, men with more 'masculine' faces should be more attractive, because there are evolutionary costs involved in developing such characteristics. The reason comes back to the testosterone levels associated with the very male face-shape. Testosterone decreases the effectiveness of the body's immune system, and so only healthy individuals with high quality immune systems can afford to produce the hormones required to produce masculine characteristics. The quality of our immune systems is linked to our genes.