This is my mane six headcanon, on both their age and height

Pinkie Pie is the youngest. She is the most immature and she doesn't seem to even have her own place. She just lives where she works. She is very pudgy and small.

Rainbow Dash is the second youngest, because she's immature I suppose. I know how everyone loves to imagine Rainbow Dash as short and sall, but I just don't. As another brony once said, short people are hard to notice. Rainbow Dash is not. And with long legs, she can take bigger stides, she can be like Zipp, she's long and nimble, and she just looks awesomer to me!

Fluttershy is the next youngest. She's already said she's a year older than Pinkie Pie. But I also think she's older than Rainbow Dash. She's in the same flight class as RD because she was held back. She also has her own place, and I dn't know if Rainbow Dash does or not, so that's a thing. I know how everyone likes to see her as being tall, but like I said, short people are hard to notice, which is exactly what Fluttershy is. Hard to notice. Anyway, I also like to see her short, as I like to imagine that as a filly she hit an early growthspirt (so she's really awkward and didn't grow into her tall legs yet) but as a mare she's really short and petite.

Twilight Sparkle is slightly older than Fluttershy. She is mature (she has to because she becomes a princess), she has her own place, and she basically raises Spike. She's the tallest, even as a unicorn (and that's totally not because I drew her tall as an alicorn but then decided not to have her as an alicorn because I hate alicorn Twilight and was too lazy to draw another pony, I don't know what you're talking about 9-9). I think it makes her look more elegant and magical.

Applejack is the second oldest. She basically runs the farm, she's very mature, and she was allowed to walk around by herself in the big city. She's about average height, also being a bit bulkier than the average mare.

Rarity s the oldest. She's the most mature, she runs her own business, yeck she seemed to be running her own business as a filly too. She's also around average height. But slightly shorter to give a petite vibe.