As a teacher with a LO due in August (got the dates wrong for the summer hols lol!) I can say that there is some correlation between exam results and age in the year...BUT!
There are many, many more factors than this which you can easily overcome in many ways. Growing up in a household where books are read/the news watched/issues talked about over the dinner table etc i.e. a household where kids are encouraged to be interested in the world, have their own opinions and know how to find out the things they want to know - is a crucial factor in producing students who want to learn and who are responsive to it.
One issue which is hard to overcome is the maturity of boys - they may well be expected to 'act their age' when they find it hard to - though I think this applies to boys of all ages through school, if it is going to apply at all. Some of the most mature, intelligent and thoughtful students are those who are boys born in July and August!