An adult who seems always to have a youthful or childlike quality, regardless of age, likely has a strong dose of Ingenue or Gamine.
Big eyes, a large forehead, a small nose, and a round or square face all help to create an impression of youthfulness.
One way to think about the seven style types is to think about the words we use to describe the type of beauty each identity embodies.
Which isn't to say Gamines aren't incredibly attractive. They are incredibly attractive. Women who have a "yang" or masculine quality to their beauty are no less attractive than their more "yin" counterparts. They only appear unlovely when they're placed in a clothing context that's more stereotypically feminine than they are.
You can see the boyish quality of a Gamine in the following ways:
Which isn't to say that Gamines need little detail. They look great with a lot of detail in the clothing -- pockets, buttons, cuffs, etc. But the jewelry is best when it's minimal. A lot of jewelry reads as feminine, and feminine context around a Gamine will make her look masculine.
The effect of clothing context on our apparent masculinity or femininity is analogous to the effect of color on our skin.
The apparent color of your skin changes, for better or worse, depending on what color is next to it. That's because of simultaneous contrast.
And the apparent qualities of your face and figure, including the apparent masculinity or femininity, change depending on the context that surrounds it.
If almost everything in the frame reads as boyish, then the viewer mainly notices what's not boyish - and so the Gamine's feminine qualities actually stand out more.
The more boyish the context, the more beautiful Gamines look.
Surround them with traditionally female decoration like long locks, ruffles, and lavish jewelry, and they become less lovely.