Board Thread:General Discussion/@comment-28431168-20160617130858/@comment-5670246-20170208220406

Souhiro wrote: This is a pet theory for me.

First of all, monsters don't get old unless they have a baby. So, the child "Drains" the energy of the parents, making them old at the same time the kid grows. That's why Toriel and Assgore looked and behave like "In their late thirthies, or early forties"

But Sans and Papyrus look and behave like old teens. Just like they didn't had fully matured. It looks like daddy (Gaster) dissapeared without a trace!!

Also, there's my SECOND pet theory: Gaster killed the Blue human (The girl who danced ballet) She was the most dangerous of the visitors and she's the only human confirmed to have killed monsters. Only Gaster was able to kill her, took her trait (Integrity) and the ability to make Blue attacks. Then he bequeathed that power to his childs, Papyrus and Sans. Actually, most monsters age without offspring. Only Boss Monsters (different from bosses; the only known boss monsters are the Dreemurrs) have to have a baby to age. Also, while likely, there is no proof that Integrity ever did ballet. Additionally, she was never confirmed to have killed monsters. It seems you didn't do your research.