I'd rather right a complaint to the state of NJ than Dicks. If the laws of NJ were a bit more lenient, it wouldn't be an issue at all. The Dicks Sporting Goods in my area would ask if the .22LR ammo I was buying was for a pistol and would sell it to me if I said no (while under 21).
The way I look at it is this: If he wasn't sure I think he had every right not too. If for *whatever* reason something went wrong, its his job on the line. Could he of asked you to wait and double checked with somebody, most certainly. I think the possibility of having a person wait and possibly still not come up with an answer may leave the customer fairly irrate.
The laws and those who enforce them are twisted. My girlfriend was working at a Wawa when she asked to see the ID of someone who was clearly older then 35. The guy got a bit belligerent when he said he wasn't going to show it to her, so she said sorry no sale. It turned out he was a secret shopper, and had she not done so could of done so, it could of been a bad thing for her and the store. If they are like that with Tobacco in PA, I can't even think of what NJ does to enforce their firearms/ammo laws.