Check it again. I think you will find they have just fixed it. I had 21 movies to redeem that I was caught by this age check glitch. I had a support call open with the iTunes store which didn't seem to be getting anywhere they wanted me to talk to apple care. However today I checked and it is working. You now get a dialogue box that pops up with those words and an ok button and everything redeems ok.
I'd say the support people were told maybe from someone else's support case and just fixed a glitch in the way the store responded back to the itunes client, as it seems the age text in the page itself was incorrect behaviour..
So anyway give it another go. I think you'll be happy. I know I am finally after much trouble shooting time spent.