Well, a legal code can both codify discrimination against children in some specific contexts, and broadly ban it in others. (It's true, the term "age discrimination" has been coopted to refer to discrimination against older people.)

For example this NYC law bans a category of discrimination against children: http://www.nytimes.com/1993/12/04/nyregion/new-york-to-bar-a.... I wonder if it's broad enough to challenge the Uber policy?