Ethically, yes, it *should* be left to the son to decide. I'd say starting at age 10.
The thing is that if you grow up in a circumcised society, leaving the son uncut through puberty can be very mean because he will be made to feel inferior/different and lose self confidence. In such cases, as much as I hate to say it, Dr Spock was right in saying you should have your son cut at birth so he will "fit in".
In a society where it is truly 50/50, then the "so he will not feel different" no longer applies. I am not talking about some statistics produced by anti circ, I am talking about real locker rooms for men of that age group. (aka: reality).
In a society where few are circumcised, then "fitting in" argument kicks in again, this time in favour of leaving him uncut.