It does depend a lot on the child.My brother and I are both July birthdays,we both did well at school.My son has already done very well at school although he's the end of May.
It also depends on the school,they should be teaching the child according to their needs and abilities.Those born at the beginning of the year can also be held backMy October daughters both do very well,but it's been a case of constant nagging the teachers,and with my 7 year old a change of school to be sure they're getting work catering to their needs,rather than her not progressing further because we don't do such and such at this stage.
Physically though kids vary a lot in size,my son born in May has always been one of the biggest,it's only complete disinterest in sport that holds him back,my son born in January is still one of the smallest,while his best friend is a good six inches taller than him and anyone else in their class.
I do think it evens out as you get older,a child develops their own natural talents and abilities.At an early stage though there can be big differences,not just because of age,some children are more mature or more able,and all children should have support to learn at whatever stage they're at.