It is very, very on the air. In the show, he is around 4 to 5 years younger than Twilight, which is the 3rd oldest of the Mane 7 (including Spike). Some staff has said his personality is around 12-14 years old because Dragons mature slower than ponies.
That is something the show rarely touched: Spike nebulous space in which he doesn't fit, age-wise, with anyone else. Everyone is either older or younger than him. Case and point, CMCs and Mane 7. Though after all the friends he's made all over the years, it doesn't end up mattering in end. But yeah, he is, JUST by year count, around 4 years younger than Rarity and the bunch.
It is very, very on the air. In the show, he is around 4 to 5 years younger than Twilight, which is
