At 18 and with a credit card you can stay at most hotels in the US. 21 tends to only be an issue in Las Vegas.
Are you coming by RV? If so parking at a hotel can be an issue because it's tight at many of the close Harbor hotels. I'd suggest the Anaheim Plaza because it has plenty of parking room. Also it's close about a 7 minute walk, and cheap too.
As far as driving, I did a couple college Spring Break, car trips that were over 1,000 miles each way. My advice is be really cautious about what everyone can spend, because it's a sensitive topic. Know the fixed costs ahead of time like, hotel and tickets. Plan a budget for food and gas well ahead of time. Also make sure the car owner gets money to cover the cost of the vehicle oil changes and insurance. Also plan a plan for incidentals while traveling, like lossing a room deposit, towing or a new tire. Lots of problem can be cause if one person gets stuck paying the tab for others, or is forced to overspend because the rest of the group wants a nicer dinner.
Also plan the schedule too. Let everyone know and agree on how many days doing what and where. It sucks but is so common that you start fighting and 2 people go one way 2 the other. It doesn't have to be super technical but when you arrive in LA to go to DL and the beach, it can quickly turn from 4 park days 1 beach day to some people wanting to save money and do 1 park day 4 beach days or voting that you leave a day early.
It appears you are aiming at Spring Break. So letting everyone in on park strategies like early hours, FP and eating at off times will help everyone relax in the parks. Don't make yourself be the autocratic leader, AKA the person who drags everyone around. Help people buy into the trip and take interest in what you will do and what will happen. Let them know it will be crowded, and figure out how everyone wants to deal with it.
On my trip we had 2 cars so we kept contact with radios, since it was before cells phones were common. Trips involved, locked keys in the car, two cars off the road, flat tire, one car wanting a different schedule, sitting in the condo all day, driving all night and driver getting on a different sleep schedule. Planning what to stop and see along the way would of helped too.