Okay, so I'm 99% positive that the skeleton in the first picture is Gaster and the goat is Asgore. While I get why Asgore might be with a human on the surface given the boss monster aging thing, how is Gaster that old? True, he probably missed some years while in the void, but still, working off the idea that the barrier was probably erected at least one hundred years ago (probably more), how is he still alive? I thought gerson says only boss monsters age based off of kids.
You answered yourself.