I'm not aware of any way to remove the age question, but within their profile settings, they don't have to have it display. As well, on your members page, there are options for birthdate/age to not be displayed if you click on the options for the member listing element. They do have to enter their age, but no one on the site will ever see it other than you, if you set things up that way.
Since there are laws on storing information on minors of a younger age, each and every website on the internet has the right to ask for age at registration, and as well should be doing so. That would be the way you'd probably want to explain it to members who have quirks about it. Not asking age because of a handful of people who have a problem with it isn't worth the headache of breaking laws and catching a lawsuit.
