Brian de Alwis <brian.de.alwis <at> usask.ca> writes:
It's a restriction required to obtain approval from our research ethics board -- people under 18 are considered to be minors in Canada and thus require the consent of their guardian to participate. Trying to obtain such permission for an anonymous survey is a bit difficult!
But since your survey is anonymous, you can't be sure all the responders are over 18. Actually, they might even not be human beings! (hint: I'm not)