Gotta die first to become a Guardian, most folks in the world prioritize the safety of children over the safety of an adult. So, while I don't see any reason it COULDN'T happen, I do see the reason it would be rather rare. One more thing, most guardians whose origins we know (I'm too lazy to quote anything do this is just my hazy memory) died performing some heroic feat like defending the people or sacrificing themselves to give others time to escape. TLDR: I don't think it's entirely impossible, just highly unlikely.