It has always been somewhat of a mystery when it comes to Spike's age. The way he was hatched by Twilight when she was a foal always meant he was much older than a "baby". Big Jim has clarified his age to hopefully put some of the debate to rest over on Twitter, especially now that we have seen an influx of dragons.
Consider him to be 16-22 in pony years, or 10-12 in dragon years. This also gives us a slight look into the age of the Mane 6 if we use Twilight's foal years. I'll let you guys brainstorm that one! Was Celestia's school post-magic kindergarten? Or did it start there?
Thanks to Samar for sending it!