I believe that Spike is about 10, while all the Mane Six are adults around 20. Because based on the CMC they seem about 10, and Twilight was like their age when she hatched Spike, so Twilight is 10 years older than Spike. So I imagine Spike is probably like 10 while Rarity is 21, so he's 11 years younger than her. But dragons mature in a very different way than ponies, so while he is 10 in pony years, he is still in a baby dragon stage, although maturity wise he seems like 11 or 12. Then once he's around 12 he starts maturing like crazy, and becomes like an adult when he's only 14. (But then again this might not be that strange because I think ponies become adults around 15, because I think Pinkie is like 16 and technically she's an adult because she lives on her own and has a job and stuff) So even though Spike's 11 years younger than Rarity, as a dragon he's like an adult, so more like he'd be only 4 years younger. Then they start dating when she's 25 and he'd be like 21, but really 14 in pony years. So then they're both adults so it's not as weird xD