For all we know he could be the same age or older. I figured Spike was being kept at a young age due to how Twilight was raising him.
 
Also assuming Twilight is at least nineteen and she was at least six in her backstory, that would make Spike rather young. Dragons live a long time so they could age slowly.

@PonyPon  
And Granny Smith in her flashback, and Shining Armor in Twi s’ flashback, and Surf & Turf, and Fanon!Amethyst Star, possibly Papermoon, Fluttershy in her flashback, Big Mac in AJ’s flashback, and Snails.

Twilight was 6 year old when Spike hatched.  
Now Twilight is 21 years old and Spike is 15.  
Spike only looks like a baby because he is not greedy like the other dragons who grow with greed. The teenage dragons are anywhere from 9-19, but are bigger than Spike because they are greedier.

Spike’s age = (Twilight’s age) - (Age she got her cutie mark).  
In my case, I say 5 years so if you know Twilight’s age, you know Spike’s.

Category—––Age (Years)  
1 Wyrmling——0–5  
2 Very young––6–15  
3 Young———16–25  
4 Juvenile—––26–50  
5 Young adult—51–100  
6 Adult–––––101–200  
7 Mature adult–201–400  
8 Old————401–600  
9 Very old—––601–800  
10 Ancient—––801–1,000  
11 Wyrm———1,001–1,200  
12 Great wyrm—1,201 or more
 
This is a table from D&D, still for me its quite good as a base for anyone wanting to work with dragon ages,it also makes sense for spike as he would probably be on the “very young” category, for the size up until dragons hit “young adult” they become medium size (a big dog size), when they become adults the go large (about the size of a horse).
 
Dunno if all this will help all of you but at least for me makes sense

Considering they stated dragons tend to take 100 year-long NAPS (100 years counts as a friggin nap for a dragon), then thoes teen dragons couldeasily be thousands of years old.
 
That is of course, if we assume dragons do age past the baby stated with time, instead of needing to give into their greed to grow.

@Background Pony  
>thinking twilight is less than 18  
nigga do you even 2011?

headcannon’d

@Takashi.0
 
This is what I always assumed when they said ‘Baby Dragon’. Though really if we look at Spike’s mental maturity he’s really not much different from Pinkie, one of the least mature Mane 6. He’s not in school like the CMC. He has a job and has been shown to go off on ‘Royal Business’ (LBYS). He’s gone along with the group in dangerous situations (TCE) and often ends up being the voice of reason (BNE, LZ). Does he have immaturity? Sure. The rest of the main cast do as well but I’d pin Spike as being a late teen with the Mane 6 as young adults.

I want a humanized Spike episode where he gets greedy and grows into an adult corporate executive.

I think calling him a “Baby” dragon is just a catch-all term for the stage of physical development he’s at.
 
Mentally, he’s obviously in the late childhood/just before teen stage at the youngest, the “baby” part is just because he’s short.

@The Twelfth Doctor Whoof  
You might want to include teen (Cadance in Twi’s flashback)

@Background Pony  
You don’t know that. Like Lium said, Pony’s ages have not been explored and never will be.
 
All we know is that the Mane 6 are clearly old enough to be living on their own, working for somepony else (RD, PP) or even running their own businesses (Rarity). Twilight, prior to “Magical Mystery Cure”, is essentially a grad student.
 
Following this train of thought, the Mane6 would likely be equivalent of a human’s mid-20s. But we don’t know that. We’ll never know that.
 
We also don’t know how much time passed between the events of “Cutie Mark Chronicles” and the pilot episode. We don’t know how old Twi actually was when she hatched Spike. The way the show works, Ponies have 4 or 5 basic “forms” – baby, filly (the CMC), adult (90%+ of ponies in the show), middle-age-ish (Mrs Cake), ancient (Granny Smith).

@Lium  
never been explored in the show itself, but dammit they are quiet adult even if they are kids. Applejack owns a farm, Rarity lives alone and has her own shop.
 
And who says they are the same age, it was said in the show that Fluttershy is older then Pinkie pie, who could be younger or older then Twilight.
 
Maybe Twilight is 16 or so, but the rest are surely older. What would be fitting since we can assume that Twilight only really “hanged out” with Celestia (waaaaay older) Shining Armor (older) and Spike (baby)

Ages have never been explored and never will be.

@Background Pony
 
>16 years old
 
Please,Get out.It is established that they are young adults a long time ago.

Given how old dragons could get, at 50 years of life, Spike could be considered a teenager. Similar to Miss Martian (young justice) who is 48 years old is still a teenager by Martian standards.

Twilight hatched him after graduating from “kindergarten” - which would make her age 4 or 5 tops. There isn’t a big age difference between her and Spike.

@Background Pony  
How dare you not make the ponies adult age? HOW DARE YOU? Nah, that’s actually a legit age.

@The Smiling Pony  
Exactly. We’ve seen Spike get bigger than his “friends” very quickly when his greed was stoked too much.

Twilight is approximatively 16 years old. A dragon who’s 8 is still a baby, duh >.>

Nah, they’re just greedier.

i guess so.