Because if you think of the default as being F!Robin marries Chrom to make Lucina, then M!Robin marries Lucina who is technically his own daughter making it Sweet Home Alabama. (Since Lucina time travels from an alternate universe there's room for there to be both a male and female Robin in Lucina's life which is something FeH even hints at in one of the very early events)

Yes, they are the same person. Which makes them the same age by default.

(Which also makes this situation very Incsety)