A: is not really right as it is not specific... "at five" could mean they start school at 0500 am.... not 5 years old... I would not use A
B: with five years is not grammatically correct, nor does it make sense;
C: Is a good answer as it is very clear what and when they are starting school
D: at five years of age? I would prefer to see it as "when they are five years of age" .. so its sort of ok...
C is the best answer.... followed by D, then A,.. but not B