I don’t believe you can age harden normal brasses significantly.
Age hardening requires an alloy in which the various componants
are not completely soluable in each other. copper and gold are
not, and neither are silver and copper. So gold/copper alloys,
like most yellow golds, and silver/copper alloys like sterling,
can be age hardened. Zinc and copper, though, I believe are
intersoluable pretty well. Not absolutely certain of that, but
pretty sure… The same can be said for pale yellow golds with
mostly silver as the alloying metal, with little copper. Silver
and gold are completely intersoluable in each other, and the
"green" gold alloys of these two not only start out softer than
copper gold alloys, but don’t age harden much at all either.
Back to brass, though, I’d note that there is a beryllium copper
alloy specifically suited to age hardening, often used for
certain types of springs. It could be that a brass might be
available with similar properties, if made especially for that
use, perhaps with beryllium or other similarly acting componant
of the alloy.
Hope this helps.