14 Year Old Self
I have long believed that consciousness grows more through duplication than modification, meaning our old selves get to rumble around in some sort of execution-constrained mental version control system. It’s one of the things I expect will someday be demonstrated as a purpose of sleep — a given instantiation can only run for so long — and nicely explains a bunch of oddities about the human condition, like regression under extreme stress and the entire sensation of nostalgia.
(Yes, I know about Mathematica, and the obtuse but brilliant HP48G. This is about a thousand times more straightforward…if someone ever ported this to a SAT-legal calculator, things would really get interesting.)