@TexasUberAlles  
Not necessarily. Luna could just decide to darken part of the moon either arbitrarily, or on a cyc—  
But she wouldn’t! That makes no sense in this universe.
 
…dammit.

@Dragonmage  
Implying her DOB is 2/01/93.  
The YTP Wiki states that Fluttershy’s DOB is 6/06/96.

This doesn’t account for the distance between the candles on the front and on The Dark Side Of The Cake on the second and third layers; if the candles wrap, there are 35 of them.
 
Obviously, it was meant as a display-from-one-side-only cake, whether in-universe by CheesePie or onscreen by the animators (since AKR didn’t specify the number of candles in the script); if, however, you assume that it’s not a birthday cake but a birthiversary cake, those extra 14 candles might be the number of years Dash has been in Ponyville, which would fit with her following Fluttershy to Ground Level at cutie markin’ age.
 
@Blacklight Shining  
They must have some kind of lunar cycle or there couldn’t be phases.

Everyone. Calm down.  
This is just a Logic sandwich we have here.

I’m 16 and my last birthday we had 20 candles on my cake so… it’s not always that

Or.  
You know.  
They just put candles on it according to the space available.  
Srsly bro.

I like the idea of rainbow being in her early thirties.  
I had her down in my headcanon that she was at least 28.

@Ponylooker  
Er, and 5 months. (Seriously? Where did I get .5 years from?)

@Background Pony Number 17  
It can’t be anything and .5 years, because it’s her birthday and she arrived in Ponyville on her birthday.

…WAIT.  
They don’t even HAVE lunar cycles! The moon is always out at night and always hidden in the day.

@Background Pony Number 17  
But then they’d have a celebration every month lunar cycle, not every year.

This is still a show for kids first, so the candles in sight is her true age.
 
She’s 21, people.

@Ponylooker  
Okay, if you want to overanalyze it…
 
OP is mistakenly assuming that the candles are arranged in layers of 1, 3, 5, 6, 8, and 10. They’re actually arranged in layers of 1, 4, 6, 8, 10, and 12. (There are actually two candles on each edge, but one is behind the other and thus not seen.) That means that there are 41 candles on the cake.
 
Now, it’s pretty sure that Rainbow Dash isn’t 41 years old. But if you remember, ponies often reckon times in “moons” rather than years. So the cake likely represents 41 moons, or 3 yrs and 5 months. Probably the time Rainbow Dash arrived in Ponyville, as that’s about 1 month before the premiere episode aired.

33? Well actually that would explain abit about her overreactions of sportsmanship. The way she always puts alot of supposed meaning and weight on events of sport. Sounds like an officious team coach.

@immblueversion  
Best explanation yet!

Or it’s just an arbitrary number of candles.

*did not

Simple: The animators wanted to show 21 candles on a 2D circular cake. That was the easiest way to depict the 21 candles because they did anticipate over-analyzing bronies using maths.

@blargh23  
Common sense in an overanalysis thread? What manner of sorcery is this?

or a much more likely possibility is that they just put a bunch of candles on to fill the spaces.

Maybe it’s a sum of her age and the number of years she lived in Ponyville.

Or, it’s just a really big cake and no cake is complete without at least a dozen candles.

@Background Pony #6492  
And her birthday.

@Crazy☾  
There, there…I still have to tag Bon Bon, Daisy, and Carrot Top to get it to add Sweetie Drops, Flower Wishes, and Golden Harvest.