How old does a child have to be to join sag or aftra?

Answer 1

Here is the SAG website. It does not specify any age:
http://www.sag.org/sagWebApp/application?origin=joinsag_template.jsp&event=bea.portal.framework.internal.refresh&pageid=JoinSAG&templateType=joinsag&portletTitle=null&contentType=null&contentSubType=How+Do+I+Qualify?&ln_idx=1

Screen Actors Guild
Hollywood: 5757 Wilshire Blvd., Los Angeles, CA 90036-3600
[phone] Main Switchboard
1-800-SAG-0767 for SAG Members outside Los Angeles

New York: 360 Madison Avenue 12th Floor, New York, New York 10017
[phone] Main Switchboard

The AFTRA website:
http://www.aftra.org/benefits/join.htm

AFTRA locals:
http://www.aftra.org/locals/llocals.htm

Answer 2

My guess is, as soon as the child begins earning money. Call SAG and AFTRA and ask THEM!

Answer 3

There's no minimum age, as far as I know, but there are other eligibility requirements. For SAG, you generally have to work as a union extra a certain number of times or be cast in a speaking role on a union show.