My son was born on the 1st of Jan 2015. I think different international schools have different cut-offs (as they follow the calendar as their home countries). But what I always hear is that Jan is the best month for a baby -- if cut-off is Dec 31, they will start a year later and be much more mature than the other kids in their class; if the cut-iff is Aug 31, they'll be in the middle of the class in terms of maturity so they shouldn't struggle as much.