I started school a year early because the educators at my kindergarten told my parents they had nothing left to teach me and another year would just be a waste, so I became the youngest guy in my class (about a year younger than everybody else). I went to something called a forest kindergarden, basically we were free range kids who stayed out all day no matter the weather, this was both hugely beneficial because it has been shown to increase not only physical skills but also intelligence, but it also meant I didn't learn how to sit still and draw and shit like that (school stuff).
when I started school my kindergarten teacher (school in denmark starts with a kindergarten class and then 1-9th grade with an optional 10th grade at the end) wanted to hold me back a year because i couldn't draw and sit still and all the other stuff the kids from a normal kindergarten was used to, my mom told my teacher to stfu and I advanced to 1st grade.
eventhough I was the youngest in my class I was the tallest and one of the strongest, I was also by far the most intelligent (not necessarily the most book-smart in every class, but the most intelligent by far). This was a huge leg up I admit, because I never had to fear being bullied or being inferior due to my age (I was also the second to hit puberty and grow hair on my balls, so again, huge leg up).
I was always super bored in school because everything was too easy, all the way through highschool, it wasn't until I started college I actually received a challenge and by then it was too late... school made me lazy (at least when it comes to school) because I learned I didn't have to pay attention or do my homework (or even show up for that matter*), I'd get an A anyway. There were like 5 borderline retarded kids in my class, 20 normal kids and then me, so it was only natural the bar was set way too low for me, unfortunatly the school didn't have much to offer me in the way of challenges, they only had programs for the dumb kids. I remember sitting in my little corner in english classes reading 10th grade books (the hardest material the shool had) while the rest of the class was watching cartoons with some fuzzy alien that eats clocks, as they were trying to learn how to count to potato.
I obviously don't know your kid, but if he's ahead of the other kids academically and if it's only because of "maturity" then you shouldn't hold him back a year, he'll will be even less challenged and end up like me (ok, bad example because I turned out fine, but still) and as for maturity, he'll be as mature as the kids he surrounds himself with.
age wont be a problem as long as you, the parent, don't make it one. I was one of the popular kids so I was of course part of the first batch to start drinking and having sex, eventhough I was a year younger than my friends my parent's recognized that physically and emotionally I was well ahead of the curve, and there were no reason to hold me back.
* from the 7th grade and until highschool I used to skip school a lot, I was an expert in forging my parents' signatures and was an epic liar, I very rarely got caught (and when i did it was usually on purpose to throw people off). Anything I wanted to be good at I became great at, lying was one of those things. I wasn't a compulsive liar or anything, it was just one of the many things I did to entertain myself, a challenge if you will.
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