What information is given about the age and history of Mori Summer in the Chuunibyo anime?

Question

The first time I finished watching through "Chuunibyou demo Koi ga Shitai!", "Chuunibyou demo Koi ga Shitai! Ren", and "Chuunibyou demo Koi ga Shitai! Movie: Take On Me", I had a faint memory of the number of years Mori Summer has existed being inconsistent. Watching it again, indeed, I found that there are various inconsistent numbers given.

What information is given about the age and history of Mori Summer in the Chuunibyo anime? And what are the qualities of the sources of that information (first-hand / second-hand / possibly faulty memory / citable from writings)?

Note that I'm purposely only asking about the anime and not the light novels because of How much does the Chuunibyou anime differ from the light novels?.

Answer 1

(I don't know if this is an exhaustive list, but I tried).

Chuunibyou demo Koi ga Shitai!

• ep4 at 12:52 read by Togashi from the Mabinogion (some page in the middle of the book) in the scene where Nibutani visits Togashi's house to retrieve it:

  I may have lost my memories, but I, a true magician, have walked this sphere of existence for five centuries.

• ep6 at 9:50 from the Mabinogion's Rainbow Manuscript, chapter three verse one, quoted based off memory by Dekomori in the scene where Isshiki drops in on her and Nibutani fighting about an earlier slide-kick:

  I, who have been watching over this world for more than four centuries, alone understand.

• ep10 at 15:58 from Suzuki presumably based off memory (one of Nibutani's classmates from her previous school) during a school festival:

  Hey! Haven't seen you since junior high! How old are you this year? 716?

Chuunibyou demo Koi ga Shitai! Ren!

• ep4 at 9:29 from Shichimiya based off memory when she reunites with Nibutani in a fast food restaurant:

  Long time no see! So you're in the same school as the Hero? And you're 1,216 years old now, right?

• ep4 at 15:00 from Dekomori based off memory when asked when she and Nibutani started getting along so well:

  Mori Summer-sama and I have been joined by the holy ribbon for over three centuries!

• ep4 at 17:42 from Nibutani as Mori Summer to Dekomori when Dekomori visits her house:

  I am Mori Summer, a mage who has lived for over four hundred years.

• ep4 at 19:59 by Dekomori from the Mabinogion chapter one, verse three, in the scene where she gives her student-council campaign speech on behalf of Nibutani:

  For four hundred years, I have observed the hearts of men through the tides of time.

• ep6 at 13:02 from one of Nibutani's classmates based off memory after Nibutani was asked if she had ever dated anyone before, and she answered "in middle school, maybe?":

  But, Nibu, back then you said you were 1,200 years old, right?

• ep8 at 14:55 from Togashi (no citation) in the scene where the gang is being prepared to battle the fake Mori Summer:

  Away with your shame! You are the Great Mage Mori Summer! You have lived for 1,200 years!

• ep8 at 16:20 by Nibutani as Mori Summer in the opening of her battle with the fake Mori Summer:

  Though I may have lost my memories, I am a true mage who has lived for the last 1,500 years.