What determines physical beauty? Well, biology for starters. Symmetry weighs heavily. Subconsciously all the hormonally charged youth are searching frantically for good traits for mating. At a certain point in life the scramble to physically hold back the aging process is either the funniest or the most poignant game in town. Depends on when you ask, I suppose.
Youth is beautiful and you realize that fact if you're lucky enough to grow old.
Old is beautiful and you realize that fact if you're really, really lucky.
Just keep loving and pray that you get good at it.