I like that explanation. I just wish that it got some development over the season. Like I said, I really liked Whittaker as the Doctor, but Thirteen felt quite static over the course of the season.

However,

RIVER: But look at you. You're young.
FOUR (wide grin, you know the one): See, this is exactly what I was saying to the Sisterhood of Karn the other day. Life doesn't begin until seven hundred and fifty!

...I really want this fic now.