The drinking age in New York is 21 and the East End enforces it harder than most of the state. Every beach club, every restaurant bar, every concert venue on the South Fork cards everyone who looks under 30 — and they card a second time at the bar. Door staff at the Talkhouse, Surf Lodge, and Gurney's are trained to spot fake IDs and they confiscate them on sight. A confiscated fake ID gets handed to the county sheriff, which is how an underage drinking charge turns into a real conversation with your parents. Plan around it: pick venues that are explicitly all-ages, or pick early-evening concerts where the door policy is a wristband rather than a bounce.
Beach parking permits are a real thing. East Hampton, Southampton, and Shelter Island all issue resident stickers, and the town lots at Main Beach, Indian Wells, and Coopers cost real money for non-residents in July and August. Teens aged 16 and older who live in town can apply for a permit in their own name, which is often cheaper than the family sticker. Bring proof of residency to town hall.
New York driver's licenses come in tiers. At 16 you can take the road test and get a junior license, which has a curfew (no driving 9pm to 5am without a licensed adult) and a passenger cap. At 17 with a completed driver's-ed course the curfew is lifted for school-to-home trips. At 18 it becomes a full unrestricted license. The curfew is not a suggestion — Suffolk County cops write tickets that stick.
Finally, the beach rules. Surfing without a leash on a guarded beach is a ticketable fine at every town beach on the South Fork — not because anyone is mean but because a loose board is a real injury risk for swimmers. Bonfires need a town permit. Open containers on any town beach will get you a fine and will get the rest of your group cleared off the sand. Lifeguards leave at 6pm on most town beaches and the rip currents after that are the real reason the signs are up. Know the hours before you pick a spot.
When in doubt: see safety for tick checks, water conditions, and lifeguard hours, and see concierge for emergency contacts, urgent-care locations, and the after-hours numbers that locals keep on a fridge magnet.