I think there's a *social* gap between 13 and 9, or 16 and 12, but the
developmental gap is surely much bigger now, between 5 and 1? Seems to
me in terms of what they can really *do* together, they'd be closer at
13 and 9, things like swimming, climbing trees, playing cards or board
games, talking about books or movies or sports, etc. Whether the
13-year-old would be *willing* to do that with the 9-year-old, now ...
that's another thing.

Incidentally, I had an almost-four-years-older brother, as well as a
lot of still older siblings, and now have two 11-year-olds and a
7-year-old. Dunno if a four years' gap just seems normal to me, or
what. If I'd had only one baby first, I might have been willing to go
for a shorter interval. I'd say the 11-year-olds each get on better
with the 7-year-old than they do with each other, really.

--Helen