One way to word it would be that because of the tools and equipment (perhaps insurance reasons), only guild members may attend. You may also like to jokingly say that this is an opportunity for folks to have quiet time without their families. Imposing an age limit can put out a mean image that I'm sure you do not want to convey. Not in the bylaws, but as a part of an info memo to potential members, you might suggest an local hourly daycare or possible babysitters (guild member's family member?). This conveys the message of wanting to be inclusive of younger members while maintaining an 'adult' atmosphere. As older members we really need to work at promoting the craft with the youngins'.