I started riding 26 years ago something that barely resembled a snowboard. Back then we modeled our riding style to the only thing we knew- a skateboarding stance. But my daughter who got on board pretty much as soon as she could keep a balance. She went through a really wide stance stage first, then around 10 started gradually narrowing it to about shoulder width, then widened it again around 12-13. I can't say for sure if it was due to developmental changes or the fact that during that time she turned into a total park rat and just preferred a wider stance because it helped her stick her landings. Perhaps both.