Not sure who they can legally do this. In the US, at age 18 you are legally an adult and can technically live on your own without parental guardianship.
What are you to do, get an older neighbor to hang out with you in order to sign it.
Also, these guys are on a strict schedule. They are not going to wait around for someone to answer the door, not have an ID, tell him to wait, and then 5 minutes later once you find/get your ID, he will give it to you.
BTW, I have ordered four items over $500 from Apple (iPad 1, Mac Mini, 17" MBP, and an iPad 2) and none of them required me showing an ID. I have also ordered a number of lower end items such as iPod Touches, Nanos, various covers from the Apple Store and again, never needed a signature. As a matter of fact, my last Apple TV did not even need a signature. I just happened to look outside and it was there on the front porch. :shrug: