The ones near us will take unbaptised children of baptised parents, attending church etc, but they are lower down the criteria list than baptised children attending church etc.
I have never seen a baptism age limit stated on the entrance criteria though - although as not catholic have never done much more than read their bumpf along with that of all the other local schools.
Is is to stop you getting them baptised just before you apply for a place rather than as a matter of course?