I started getting freckles at about the age of 4 or 5. Freckles are all about sun exposure though- I'm from NZ and come from an outdoors-loving family so was bound to start getting them quite early. Although my parents put sunblock on me there wasn't the knowledge or range of sun-protection products there is now, so I had probably had at least a couple of sunburns by then.
Basically the more you protect your LO from the sun the longer he will go without getting freckles, although if he's prone, they'll no doubt appear at some point.