Why does the Buddha blame Ananda asking him to remain living forever? in the Digha Nikaya 16 The Buddhas Final Nibbhana , the Buddha blames Ananda for not requesting that the Buddha live for aeons, three times.  he had only asked him twice.  Why does the Buddha blame him ? And the Blessed One answered, saying: "Enough, Ananda. Do not entreat the Tathagata, for the time is past, Ananda, for such an entreaty."


  50-51. But for a second and a third time, the Venerable Ananda said to the Blessed One: "May the Blessed One remain, O Lord! May the Happy One remain, O Lord, throughout the world-period, for the welfare and happiness of the multitude, out of compassion for the world, for the benefit, well being, and happiness of gods and men!"
  
  
  Then the Blessed One said: "Do you have faith, Ananda, in the Enlightenment of the Tathagata?" And the Venerable Ananda replied: "Yes, O Lord, I do."
  
  
  "Then how, Ananda, can you persist against the Tathagata even up to the third time?"
  
  
  Then the Venerable Ananda said: "This, O Lord, I have heard and learned from the Blessed One himself when the Blessed One said to me: 'Whosoever, Ananda, has developed, practiced, employed, strengthened, maintained, scrutinized, and brought to perfection the four constituents of psychic power could, if he so desired, remain throughout a world-period or until the end of it. The Tathagata, Ananda, has done so. Therefore the Tathagata could, if he so desired, remain throughout a world-period or until the end of it.'"
  "And did you believe it, Ananda?"
  
  
  "Yes, O Lord, I did."
  
  "Then, Ananda, the fault is yours. Herein have you failed, inasmuch as you were unable to grasp the plain suggestion, the significant prompting given by the Tathagata, and you did not then entreat the Tathagata to remain. For if you had done so, Ananda, twice the Tathagata might have declined, but the third time he would have consented. Therefore, Ananda, the fault is yours; herein have you failed.
aeons


It's not aeons, not living forever. It's living only until Buddha's old 100 years. 

Rhys David:


  kappa (adj. n.), anything made with a definite object in view


So, the translation should be something like this:


  "O Lord!  May the Blessed One remain, O Lord! May the Happy One remain, O Lord, throughout the defined median age (Buddha defined it in DN14, AN 7.70).


Kappa in  DN 16 (Kappāvasesaṃ) means the defined median age of Gotama Buddha.

Kappa in AN 5.23 (PubbenivāsānussatiÑāṇa) means the defined age of the universe's occurrence cycle (from Brāhma's Veda which derived from Anāgāmi Brahma of KassapaBuddha, previous Buddha).

Kappa in AN 4.156 (AsaṅkheyyaKappa) means the defined uncountable years of each procedure in the universe's occurrence cycle (4 procedures per 1 cycle).

See the defined median age in Tipitaka:

DN Mahāvaggo MahānidānaSutta:


  Vipassī lived for 80,000 years.  Sikhī lived for 70,000 years.  Vessabhū lived for 60,000 years.  Kakusandha lived for 40,000 years.  Koṇāgamana lived for 30,000 years.  Kassapa lived for 20,000 years.  For me these days the life-span is short, brief, and fleeting. A long-lived person lives for a century or a little more. 


KN Buddhavaṃsa, 24 Buddhas foresaw:


  Gotama Buddha's age is 100 years old.


KN Buddhavaṃsa, our Buddha said:


  Even though my age is 100 years old and lesser than the other Buddha, I can let many people enlightened, too.





***Search "vassasataṃ" and "vassa sataṃ" here. I will not post the search result direct link because it maybe overuses of a website resource.*

Atthakatha described the same.

  the Buddha blames Ananda for not requesting that the Buddha lives until touch the defined median age, three times. he had only asked him twice. 


Buddha asked Ānanda only 1 time per question after Ānada request the Buddha three times. Buddha's questions are: 


Do you have faith, Ananda, in the Enlightenment of the Tathagata (who has already renounced his will to live on)?
Then how, Ananda (who accepted in the Enlightenment of the Tathagata), can you persist against the Tathagata even up to the third time?
And did you believe in what you said that you have heard and learned from the Blessed One "'Whosoever, Ananda, has developed, practiced, employed, strengthened, maintained, scrutinized, and brought to perfection the four constituents of psychic power could, if he so desired, remain throughout a defined median or until the end of it. The Tathagata, Ananda, has done so. Therefore the Tathagata could, if he so desired, remain throughout a defined median or until the end of it.'"", Ananda?
So, they are not the same question.


  Why does the Buddha blame him ? 


Buddha blames him in the second question because of the first question. Ānanda already memorized DN29, so he knows that the Buddha can't change his decision renouncement. If the Buddha didn't ask him, people will misunderstand "Ānanda, Sotāpanna, can doubt in Buddha". But after Buddha asked b.h.Ānanda by the second question, people will understand right "Ānanda, Sotāpanna, didn't ask Buddha because of the doubt, but he asks because he trusts in both Buddha's speeches (renouncement and DN29) and tried to let Buddha does follow the DN29". By the second question, people will not misunderstand in Ānanda. 


See DN29:


  From the night when the Tathagata understands the supreme perfect awakening until the night he becomes fully extinguished—through the natural principle of extinguishment, without anything left over—everything he speaks, says, and expresses is real, not otherwise.
  
  That’s why he’s called the ‘Tathagata’.






  And the Blessed One answered,
  
  saying: "Enough, Ananda. Do not entreat the Tathagata, for the time
  
  is past, Ananda, for such an entreaty."


As I quoted DN29 above that Buddha can't change the renouncement.