That's also true. When I went to school in the USA they use to put kids born in December with the kids born through the rest of the year but that stopped 10 or so years ago here and now it's not true. And we don't know if they're going by the Japanese calendar either. For one thing we don't seem to get any seasons in Geass despite Japan having rather obvious ones lol

We just don't know, I think in the end it's just not that important necessarily for the plot and something we as fans can sort of decide on. ^_^;; I personally see Suzaku on the younger side and Lelouch on the older side but that's just me lol