Booking accommodation under 18 years old, adelaide australia?

I don't know about the beach houses because they usually are private and it depends on the owner of the house but talking about the hotel rooms, for sure you have to be at least 18 years old to allow you to stay in the hotel! If you decide to stay in a hotel, you can ask some of your friends or relatives who are at least 18 to make the booking for you! But it is advisable for the person who makes the booking to accompany you because at the reception desk they will ask him/her to prove that he is the one that make the booking and to prove that he is the owner of the credit card! This is the best you can do for now! And about the booking itself, you can search for hotels in various websites such as expedia.com, hitrooms.com, booking.com! they will offer you a wide range of nice hotel accommodation offers and you can easily make your decision! so you can check if you are interested and decide what to do!