Enfranchising incompetents: Suretyship and the joint authorship of laws

Proposals to lower the age of voting, to 15 for example, are regularly met with worries that people that age are not sufficiently 'competent'. Notice however that we allow people that age to sign binding legal contracts, provided that those contracts are co-signed by their parents. Notice, further, that in a democracy voters are collectively 'joint authors' of the laws that they enact. Enfranchising some less competent voters is no worry, the Condorcet Jury Theorem assures us, so long as the electorate's competence averaging across all voters remains better-than-random.