Hotels you can book at 18 in Arizona
If you are 18 and headed for Arizona, some hotels will require you to be 21 at check-in, and it is better to find that out while you are still choosing than after you arrive. The Arizona city covered here has hotels that permit check-in at age 18. Once you have that answer, the next thing to plan for is the heat.
19 hotels
Phoenix
Plan your dates and your room around Arizona's heat
The covered city is Phoenix, one of the hottest, sunniest cities in the US, and during Arizona's hottest months the National Weather Service issues heat alerts when unusually hot weather is expected. Heat is Arizona's deadliest weather, which is a reason to plan for it rather than a reason to cancel. If your dates are flexible, the easiest call is to keep them out of those hottest months. If they are fixed, the planning moves into the room itself.
For a stay booked into that stretch of the year, air conditioning is the first thing to confirm about a room, before the rate and before the photos. Location deserves the same scrutiny, because a hotel near whatever brought you to Phoenix beats a cheaper one that leaves you walking long stretches in the midday sun. Book so that the hottest part of the day is something you can choose to sit out.