should control arms be replaced in pairs?
 

Wheel alignment shop told me that ball joint on lower left front control arm is worn and needs replacing. Is it OK to replace control arm (ball joint cannot be replaced by itself) only on one side, or should one assume that if one side is bad the other will go soon?

The other side is not far behind. If you have the $$ now, replace them both. If not, it will do no harm-and you will never notice- if you do only one side at a time. Just make sure alignment is right on.