Setting user Gender & Age in Google Analytics iOS

Question

Using the latest Google Analytics SDK (v3.0.6), and I can't find anywhere in documentation, searching through the header files or web search how to set the Age and Gender of the current user for GA tracking.

I'm using Custom Dimensions successfully for other things, and could use two of those for Age and Gender, but it doesn't seem like what they're intended for when there's built-in "Dimensions" for Age and Gender already.

I imagine that it would be something like this:

[tracker set:[GAIFields customDimensionForIndex:4] value:@"Gender"];

But doesn't seem to be anything in GAIFields.h or GAIDictionaryBuilder.h to set the built-in dimensions.