This is untrue in any state in the US. Possibly England or some other country. Alcohol is legal (to posses and drink, not buy) in some states, some situations for 18 yos and even a 13 yo at their Bar Mitzvah.
A retailer as Bruce said reserves right to refuse service to anyone. Perhaps One shop refused? One Wal Mart refused sale of 9mms to Loughner; the next sold him a box of 20. He shot ten when firing on Gabby Giffords later that day.
In my state, even a felon is allowed a pocket knife where extended, blade + handle does not exceed length of palm. A very nice AZ Ranger gave me lift to Globe, he saw my little Swiss Army pocket knife and Laughed.
Automatic knives are legal in my state.* So are Big Bowies, machetes, swords, etc. I think these are restricted to 18 yos as they can be very dangerous, you know how 10 year olds play with things. If one lost an eye playing with a switchblade, the liability.
+ So are automatic weapons ("machine guns") Very restricted though. I've shot a BAR, STEN, Maxim etc, Legally in Marana.