You can't really complain about the Ember x Spike age difference when rarity or any of the mane 6 are much older than Spike. The mane 6 are virtually adults, this is a definite. Also, the show hasn't confirmed the age of Ember, for all we she could be the right age for spike. She's still practically a 'daddy's girl' I would say she's quite young.
Personally I'm glad they introduced Ember not really as Spike's love interest, but a dragon friend he can relate to (not just pony friends) Although I still feel Spike x Ember ships are far more tolerable than Spike x (pony) ships.