I think that Spinies don't have shells. They just keep the egg they hatched from on their back until their real shell grows...
Unlike the eggs, these shells are even fireproof!
I mean, Super Mario Land doesn't have anything to imply that Mario and Daisy don't know each other (unless I'm forgetting something, which is entirely possible).
Anyways, I see Mario and Luigi as around 25, since it's in between both of the previous stated ages (26 and 24). Given this...
I have two. The first is that Toadsworth is Peach's father, and this is because I remember a YouTube video say that the European SMS website claimed this. The two already had a close relationship, so this could make at least some sense. My other one is that the Koopalings are Bowser's children...