1) We're just more vocal about our birthdays because we're used to people being in the party mood--I said elsewhere that I can't go out for a birthday drink without finding all venues filled with tinsel etc...

That could be proved/disproved by a poll: "Which month were you born in?"

2)  For different characteristics due to "time of year", yeah I think that's interesting.  There are a lot of different factors (I think first of "When you took your first steps, was it warm and sunny, cold and snowy, blowy and wet?", all the seasonal variations--but yeah, how about the school year in England runs Sept-Aug, so Sept = older kid, Aug = younger kid for any school peer group; whereas the italian year runs (or ran) Jan-Dec, so in Italy (hey, we're both two hundred and thirty nine, right?) you'd be the oldest in your class and I'd be the youngest in the class above you, whereas in England we'd be close (almost the same birthday!) in the same grade.

Don't fight forces, use them R. Buckminster Fuller.