Random Headcanon: Spike's wings · 2:19am Nov 4th, 2015
I believe that the reason why in both instances of Spike physically growing up prematurely; magically during Twilight's loss of control over her magic before gaining her cutie mark and naturally during Spike's loss of control over his greed in Secret of my Excess; he did not grow wings, is because wing growth is the one aspect of dragon aging that cannot be rushed, like cutie marks. If I were to speculate further, I'd say as a result of this revelation, wing size is the only reliable way of determining a dragon's general age.
Maybe...
Or maybe he's a wingless dragon.