From
http://www.i18nguy.com/l10n/shoes-anatomy.html
"There are also lots of wisecracks in the discussion forums about shoe size not being important, it's the width that matters. (It's even harder to find data on shoe width ..."
"Your feet continue to grow, so age makes a difference."
One size larger isn't that much of a difference for me to argue. But if age makes a difference, then growing feet make a difference. If growing feet make a difference, then my advice to buy shoes one size larger is better than the other advice given to buy a shoe that fits.