Unless you count the other blue mask kids.....

On my game they were adults or at least close to that.

I've always had a few blue mask kids in my few games, and as far as I can tell, heathens don't age.

Your right they dont age. I converted the heathen farmer when he was about twenty years old. The rascal was I'm guessing around the age of eight. He is now fifty two and the rascal that I converted yesterday was still a kid and he was the age of eight. You could have someone be born live to te age of one hundred and he would still be a kid.

I had many blue mask kids because of the Barrel Of Babies event where the babies choose the heathens side instead of the believers side. I don't think I had blue mask kids in any of my five games at the very start.

I've also gotten many blue mask kids from the events where my villagers get turned into heathens.

Maybe it's random? *shrugs*

The devs would know whether there are suppoused to be blue mask heathen kids at the start every time or it's only sometimes.