fencing?  There is no right age.  In order to participate in national level fencing
tournaments, children must be at least 8 years old.  This is the youngest that we
suggest that kids join our group classes.  We have specific 10 and under classes.  
If you wish to have your child take private lessons that will be up to the instructor
at your school and their availability.
There are the following age restricted events at a national level.
10 and under (8 is the minimum)
12 and under
14 and under
cadet (17 and under)
junior (20 and under)