Dragons live an extensive amount of time longer than ponies. All of the mane six will start to die from old age one by one but Twilight will outlive them all and Spike will be there until she leaves him too.
people have factored in a detailed timeline of ponyville and stuff when Ponyville would have to have started and put that in perspective for when Granny Smith was young in her flashback. She turned out to be around 300 years old according to popular theory.
Well, she helped found Ponyville as a filly right? Twilight told Spike that Ponyville had been doing Winter Wrapup for hundreds of years. This implies that she's at least over 200, and normally hundreds implies over 200 alone.