We can't allow infants to vote. That seems to be your premise even though it is written after instead of before your conclusion. Obviously, some age limit should be set. This conclusion drawn from your premise is not obvious to me at all. Please educate this Starbucks barista who flunked out of college. What is the reason for not wanting infants to vote? Is it the infant's chronological age? If so I could see how your conclusion could follow your premise. I would assert, however, that it is the cognitive capacity (or lack thereof) of the infant that is at issue here--NOT it's chonological age. Therefore, the question does not become What should that age limit be? It becomes, what level of cognitive capacity is required for an individual to "deserve" the right to vote?
Okay. Now what is your idea for determining the "cognitive capacity" of each member who signs up on our website? 
Minimum Cognitive Ability Required to Sign the Statement of Intent=Enough to be Capable of Expressing a Desire to Participate
I'm willing to compromise, though. How about:
Minimum Cognitive Ability Required to Sign the Statement of Intent=Enough to be Capable of Eloquently Providing a Reasonable Assurance of Casting a Well Reasoned, Serious Vote
This will weed out the 12 year olds and most teenagers I know, but not GimmeFuel. (Well, maybe a really, really, really exceptional 12 year old...)
I would like a proponent of the age limit to address the issue brought up earlier about the way this policy encourages dishonesty. I believe it was rhull who pointed out that all GimmeFuel (who is male by his own assertion) could have just kept his mouth shut. This point is especially troubling to me since Jason later admitted that admission policy is de facto "don't ask don't tell". I am familiar with the primary effect of the original "Don't Ask Don't Tell" policy. Having lived on an aircraft carrier, I can assure you that it was an excuse to let enlistees gay bash. Everywhere I went all I heard was faggot this, faggot that, let's go beat up some faggots, etc. But god forbid you glance sideways at a black, a woman, or an asian american. You would be in front of the captain facing restriction and pay reductions. It was impossible to get in trouble for gay bashing as a direct result of the "don't ask don't tell" policy, and the policy was supposed to help gays and lesbians. And I'm not talking about just my shop. This is on a ship with 6,000 sailors and marines.
Now Jason, I'm afraid I must insist on pinning you down on something. Is your insistence on a minimum age a philisophical position (along the lines of, "disallowing minors to vote is an archtypal example of a policy that reflects libertarian ideology) or is it a pragmatic/logistical issue? You have mentioned previous problems, but you have not responded to my suggestion for handling similar problems in the future.
"So you had a problem with teenagers and parents disagreeing on whether the teens should be members or not. Interesting that you did not mention which side of the debate the kids and parents were on. Did the kids want to join and the parents refused permission? Or were the parents trying to force their kids to sign to get more votes?"You haven't addressed these questions, or indicated how prevalent those situations have been.
You also mention the text of the ballots that have already been printed. Here is a cost free solution: Have the board unanimously vote that the wording is a mistake and that it is okay to cross it out before signing and submitting the ballot. The decision will be based on the fact that by adding wording that does not appear on the statement of intent or the participation guidelines to the ballot, I have the right, as do all members, to "withdraw my consent" since it is, in effect, a change in participation guidelines.
I will submit my ballot with the words "that I am at least 18 years of age" crossed out. If you do not count my and GimmeFuel's ballots, I reserve my right to withdraw my consent.
One more issue, I would like to hear from the rest of the board. Is this a unanimous position? Are you out there, Tim? I know you are not bashful. I saw that government worker thread.