Yeah someone over 18 has to sign for you. Only for your first visit though. after that they got you on file unless you don't come for a really really long time, guessing on the second part. give him a call and talk to him they're pretty cool you might be able to pick up the waiver ahead of time and then bring it in when you come to skate.
When I was there I signed a physical waiver; however, you will most likely need a parent's signature if you're under 18
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