Oh, I don't know-- it depends upon the child, I think.  DD saw a production of 
Shrew as her very first live play (community theater, but still) when she was six.  Yes, the humor is very bawdy, and the battle-of-the-sexes, firmly tongue-in-cheek gender role stuff in the play required a LOT of talking, but--
she handled it fine and really enjoyed the play.  It's a lovely intro because the comedy is 
so over-the-top and physical in a production of it.  Very much like Three Stooges or Marx brothers comedies.
She also, um...  identifies rather strongly with Katerina, which no doubt helped.  
