Considering the time they live in, they rely on the priority's of other ponies. Kind of like the 16th centry. Even to their present day I would have to agree with the age of possibly 80's. However some ponies live longer than others... Sort of like Celestia, she was still the princess when granny smith was just a filly. Yet she's still very long, maybe due to royal blood though is what makes her live for so long... but yes. 80-90's... maybe even 200. xD
Hmm, most of this irregularities can be explained by the relationship of time. The citizens of Equestria may have a different day - year system than us. Celestia controls the day, and if we go as 365.25 days every year... Celestia may have extended the day or decided to wake up late, therefore skewing the reference of time from "Earth" to "Equestria". Hundreds of years? That is a minimum of 200, but we don't know how long a year is in Equestria also... So Granny Smith can be a probable 50-80 earth years old.
