It's not always about looks either - it may also be about how she carries herself, her demeanor, her attitude, etc. Sometimes youthfulness is intangible, and difficult to cultivate, and it ain't necessarily something that the youthful person is even aware of.
I've been told this (about myself, i'm 39) all my life. It's often been frustrating, weird, flattering....but I'm mostly nonplussed. I don't dress or act younger, and I can sure as hell see the changes that have occurred in my face, skin and body over the years, but people still make some pretty weird comments about it.
Can also be about face shape and features: larger eyes, lip shape, etc.
I don't feel like I'm explaining this very well, but I guess I'm saying when I've noticed this in other people it isn't something clothes and makeup seem to alter. Nor is it about weight. Some of my relatives who are in their 70's look much younger than they are, and it isn't down to clean lifestlyes and slenderness! Genetics, perhaps.
Anyway, enjoy 43, nothing wrong with looking that age!