As a retired programmer, I can tell you why they do that. They probably don't store that date, as the system doesn't know who you are until you pay, sometimes not then. They do it for liability protection. You have to be over 18, and they want that verified by computer math, not some clerk. Otherwise, that clerk might let underage friends buy ammo. Yes, those friends could lie, but that lets the store off the hook. It's all about CYA.
I once asked a clerk at Cabelas if he couldn't tell I was way over 18, and he said, "yes, but my terminal can't."
This is the world we live in.
![anamatedbannana :anamatedbanana](./images/smilies/anim_bannana.gif)