okay, according to Lauren Faust, ponies age the same way that humans do. The mane 6 are somewhere in the range of 17-19 years old. Dragons on the other hand, age very differently. They typically live a very long time so their aging process shows that. Dragons normally stay considered a baby (like Spike) for several hundred years before advancing to teens (like Garble and Ember). They spend a long time as teens and when they finally become adults, they can stay that way for thousands of years. After all, in "Dragonshy" Twilight did say this, "Equestria simply can't survive a thousand years in a dark haze." A thousand years. If they take thousand year naps, just think of how long they live! Also, in terms of theory since Spike was not born like normal dragons, he might not age the same as them. As he was hatched from Twilight's magic, he may have been affected.