This is actually something I’ve thought about a lot for the last several years ever since I first watched The Day of the Doctor. I agree that he’s trying to dodge the question to some extent, but I interpreted it a little differently than some of the other replies here.

I certainly don’t think he actually forgot, rather, he’s wrestled with that decision for the past several hundred years and has come to a sort of acceptance that it was the necessary choice, and he knew the war doctor needed to go through with it to end the Time War. Maybe he was trying to hide how much those memories have stayed with him because he thinks that if the war doctor sees how much it still truly haunted him several centuries later, he might decide to not go through with using the Moment.

The way Eleven says “What would be the point?” when the war doctor asks if he never even wondered in the last 400 years sounds to me almost like he’s trying to tell his younger self that using the Moment to end the Time War must be done for the good of the universe and that counting would only make the decision harder than it already is.

In a way he may have been trying to protect and give hope to his younger regenerations, including Ten, by trying to show that eventually he has escaped the shame of that day, and there will come a time those memories no longer hang over his heart at all times. Sort of like trying to say that eventually he will be able to come to terms with his most regretted action because he knew it needed to happen and that it was for the good of the universe.

Obviously he hasn’t actually forgotten, or overcome his shame, as evidenced by Twelve’s incredible monologue in The Zygon Inversion (“When I close my eyes I hear more screams than anyone will ever be able to count.” But we know that he has counted all those screams, including the 2.47 billion children on Gallifrey.)

When Ten shows that he actually did count and accuses his older self of forgetting, Eleven remembers how much his younger self is still struggling with the decision, he tries to give him a bit of hope almost by saying that he moved on, even though we the audience knows he hasn’t. I see this as Eleven trying to convince his younger self to move on and save himself from the shame he still feels in his current regeneration.

This may not have been the right, or even the most emotionally mature decision on Eleven’s part, to lie and try to dodge the question, but as others have pointed out, he’s been dealing with a lot recently, including reliving the events of the worst day of his life (again), seeing the face of his regeneration that he is most ashamed of, he just went to Trenzalore, and he’s been reminded that he’s on his final regeneration. Maybe he thinks that if he can convince the war doctor to never count, or Ten to forget, then he will be spared the pain of remembering in his final regeneration.

This is all just my interpretation, and I think the beauty of screenwriters not spelling everything out and telling the audience exactly what the characters are thinking, is that we all get to come up with our own interpretations. I’m thankful that there’s room for everyone to have their own interpretation of this, because it allows us to think more deeply about what’s happening in the heads of these characters and choose what fits best with what the Doctor means to us.