I'm fair-skinned with plenty of freckles, my oldest DD was born without any freckles but has been developing them slowly over the years. She got her first couple by the time she was 9 months, I think. (She's 4 1/2 now.)
My second DD was born with a dark freckle on one thigh, and we're just waiting for others to show up - she's just turned 9 months and has one other, I think.
Have to go check now... = )
I don't think they are all activated by sunlight (witness my newest being born with one), but sun certainly speeds the process. = )