What makes MLP fans think Fluttershy is the oldest out of the Mane Six?

NOTE: I DON'T HATE FLUTTERSHY, SHE'S MY FAVORITE CHARACTER BECAUSE I CAN RELATE TO HER. I'M JUST CURIOUS OF HER AGE DESCRIPTION.

I recently got addicted into My Little Pony because I kinda want to switch my stress and mental feelings around... Anyways, I heard fans always say that out of all Mane Six (Twilight Sparkles, Rainbow Dash, Fluttershy, Applejack, Pinkie Pie and Rarity obviously) Fluttershy is considered to be one of the oldest. I don't get it, other than living alone, caring for animals and a year older than Pinkie doesn't make her the oldest. She sometimes can't stand up for herself and is even scared of her own shadow at first, she (sometimes) needs her friends encouragement in order to get what needs to be done. If she's older than the rest of the Mane Six shouldn't she always try to calm her friends down with courage instead of speaking in a soft voice. Rarity looks older than the rest because she can make her own business and put effort into what needs to be done. Twilight too is extremely smart and she can take care of herself easily, she's always there to bring everyone together and is responsible, doesn't she act maturely?

I'm not saying Fluttershy is bad or anything (Read the note above) but how do fans think she's one of the oldest in the Mane Six.

Thanks in advance :)