Copy/pasted from a reply below: "
(Cadence's ascension is) covered in a short novel called "Twilight Sparkle and the Crystal Heart Spell"... I'd read it before I made this. It doesn't necessarily contradict this drawing as it doesn't say at what age Cadance ascended. She could have been young enough to still need flight lessons or at least help with certain techniques. She could also be behind most pegasi in this area as she was raised primarily by earth ponies before Celestia adopted her as her niece. "
Theory of a probably-not-canon book aside, I'm glad you like it.