5 by january 1 for school starting in fall is arguably depressing education by delaying entry for 8 months, unless you've got public preschool too, 4 by jan 1 is advancing the average entry age by four months, so you might have to adjust your education standard accordingly. These are fairly arbitrary things though, and easily adjustable, but it means that student who start with 5 by 1 sept are going to be 18 before their first semester of college begins, which is probably preferable for obvious reasons, having 17 year olds on campus represents some noticeable difficulties, which is a hassle alreayd for those who skip grades but widespread quantities of students doing their first semester as minors is probably better off avoided. So I'd say a fall cutoff date is probably the better alternative, at least in the states, especially since a lot of states are pushing for 18 as the driving age, saves a lot of potential headaches for what is otherwise a purely arbitrary system.