I personally don't think the 3-month cutoff had such an influence in the amount of votes. We got 1 (one) email about someone who was interested in voting and his garage.maemo.org account was not 3 months old (but his tmo one was). I'm pretty sure that we would have gotten quite an extra bunch of emails like that one if a majority of current tmo users would have wanted to vote.
Maybe a way would be where new members can only post in only certain topics/categories until reaching a minimal karma?