I like it, though I'm not exactly a fan of seeing Spike grow up. I kind of want him to stay his size forever! d: (Even though when he grows up he loses that bit of charm he had as a baby, and possibly he's more mature than he was back then as well.)
But this is really cute and I think you made it work really well!
You know, it kind of gives me an idea. What if this Spike and the one from the show had to compete on winning over Rarity. Where as older Spike is much more gentlemanly and mature, the younger one is much more bold with his attempts and ruins the older one's plans whenever he can. I could see a really good fan-work in that!