i also believed corn starch kept indefinitely [sealed in tupperware] until i scooped a tablespoon and saw it had tiny white bugs in it. we've since relied only on arrowroot and agar to do our thickening.

cornstaich doesn't last very long in my house. i keep a small container in the kitchen for thickening and a very large container which i pour into empty powder cans which i use for bath powder. it prevents chafing and fungus and sweaty creases.