Is it okay for an eleven year old to use twelve year old mouthwash?

In most cases, yes. If it is listerine or another large brand, they are able to. If it is prescribed by a dental hygenest, then only the person it was prescribed to may use it. The reason that it usually says "12 and over" is because that is the average age where teeth are almost fully developed and old enough where kids can rinse and not swallow. Hope this helps, Sincerely, Ana