The limit means staff can refuse to serve drinks to customers who are with children.
The company said the policy made commercial sense as most of its customers are not with children and do not want them running around pubs.
"What we don't want, and what our customers who don't have children don't want, is that parents sit there for hours on end while their kids are getting bored, running round," a spokesman said.
"We will not let you come and use our pub with children because you haven't got a babysitter," he added.
The pub chain JD Wetherspoon is known for acting independently – it became one of the first pub groups to ban smoking in some outlets, two years ahead of the government-enforced ban last year, and in 2001 used beer mats to campaign against Britain adopting the euro.
All 683 Wetherspoon pubs, which serve soft drinks and food as well as alcohol, allow children to enter if they and an accompanying adult are eating, unlike some British pubs and bars which ban children entirely.