@Phantom Rider  
That’s because it’s not at all clear that she is. The whole idea, as far as I can tell, is based on a single line by Twilight in “Winter Wrap-Up” where she says: “No Spike, Ponyville was started by Earth ponies, so for hundreds of years they’ve never used magic to clean up winter. It’s traditional.” People combine this with Granny Smith being around when Ponyville was founded, and presto – they think she’s hundreds of years old.
 
However. Nothing in the show suggests that she’s supposed to be super-old, as opposed to just plain “old” – for one thing, she’s clearly implied to be the real grandmother to AJ, Mac and Applebloom, and that the history of Ponyville is not very many generations long. (There’s a clear 19th-century vibe to Granny’s flashbacks to her childhood, too.)
 
So I’m dismissing the whole idea of Granny Smith being somehow centuries old as a continuity error, created when the showrunners changed their mind about how old Ponyville is. If you want in-setting explanations for the discrepancy, perhaps Twilight was talking about Earth pony traditions in general; or (as I like to think) Ponyville as we know it is less than a hundred years old but there have been numerous towns in the same place going back centuries, all part of Celestia’s plan to make sure there’s a town to serve as a base for the future element-bearers once Nightmare Moon returns.

Nobody seems to take much notice of Granny Smith being centuries old.

@Pepsi Twist  
The comics do that with many characters. Though, some like Celestia were very controversial.

I noticed the comics give a little more screentime for Silver than the show.

Silver Spoon is a smart pone.

this was general culture, bringed to you thanks to: silver spoon

@Beau Skunky  
Silver seems like the kind of a pony who appreciates tradition and ties to the past. Di just couldn’t get past her personal animosity with Applebloom.

She likes Granny Smith now? Well, makes sense after “Family Appreciation Day.”

The Toad/Goomba/red griffon finds this interesting.