Did you even bother to follow the link? Here, I extracted the reference for you [1].
"Chinese and Japanese populations typically lose between 80 and 90 percent of their ability to digest lactose within three to four years of weaning." (Swagerty et al, 2002)
Ergo, lactose intolerance is a factor in children in some genetic groups. Your citation (sourced from the same wikipedia page) does not refute that.
[1] http://en.wikipedia.org/w/index.php?title=Lactose_intoleranc...
Relevant quote - emphasis mine: "Primary lactase deficiency is genetic, only affects adults and is caused by the absence of a lactase persistence allele.[9][10] It is the most common cause of lactose intolerance as a majority of the world's population lacks these alleles.[11]"
edit: Also, I can't believe you linked to some forum called "The Straight Dope" as your citation.