Mentioning the age thing and Lauren Faust's comments, she did say it's sort of imprecise. I thought I heard somewhere that Twilight is 5 years older than Spike, so I don't think they age like real life horses. Which would kind of suck, having such short lives and all...
Maybe they live as long as most magical creatures typically do, or like humans, as I imagined. Not deterring the whole age debate between the characters, just my opinion on their possible life spans.
Of course they age differently, but keep in mind Spike is a dragon and they don't exactly grow up fast, despite him having the the mentality of a very matured preteen. The only year estimate I would imagine them using is human years. But that's all just my (most desired) assumption.