A theory (or at least, an hypothesis)

[floriatosca]

So, along with being a video game auteur, Hideo Kojima has a reputation among his fanbase as being a bit of a troll, and liking to mess with people's heads generally. And the Metal Gear Solid series is, along with weird bosses, talkyness, and homoerotic subtext, filled with implicit commentary on various entertainment tropes.
I propose that all the gerontophile-bait fanservice in MGS4 was, among other things, Kojima teasing Japanese popular culture in general for its fixation on youthfulness.