From my understanding family members/agents age at the end of every winter turn. So running at .5 will be fine but running at 1 might cause problems.

I have modded my game enough to run at .5 and am currently (when I get free time *sigh*) testing it. I've not adjusted building times or cash income at all, purely altered population growth levels and the caps required to upgrade cities/castles in the hope that it will stop citadels/very large cities from appearing too early. Unit speed has also been left untouched as I feel it is reasonable that an army can only march so far in 6 months.

I'm still testing it out so we'll so how I go.

The whole reason for my modding the game was that I wanted to run at a 6 month set, plus it just looked weird only having winter every 4 years.