Age and sex are already features for the written language. See above (E.1.1.*.1.): between the ages of 16 and 60 broad indications of decade will do; works by children and Senior Citizens should be marked as such since they are known to differ linguistically from mainstream adult text.
Demographic coverage is important for pronunciation study but marginal in a normative reference corpus. It can be attached to E.1.1.*.3. language background.