You will find it difficult to reserve a hotel room in Las Vegas if you are 18–21, and it is next to impossible to reserve a room in your own name if you are under 18 anywhere in the U.S.
Vegas.com, the site operated by the tourism bureau, is not comprehensive, but includes minimum age requirements and seems to be reasonably reliable. The only hotel it lists on the Strip renting to 18+ is the Travelodge Center Strip, and the only off-Strip hotel is the Westgate, which is a distance away near the Convention Center. The policies page for Hilton Grand Vacations suggests that they are another option. Note that policies are always subject to change, so these properties may no longer be options come August, whereas others might be.
You may have additional options if you are willing to stay downtown or in other parts of the city, although public transportation is costly (the SDX bus is $6 for a single ride or $8 for a daily pass).
The only other listing of hotels I was able to find was at booking site LasVegasHotel.com, but this list is not reliable, as it lists examples which the official site states are 21+ (e.g. Skylofts at MGM Grand) and moreover lists the Riviera, which was demolished almost three years ago.
There are alternatives to hotels, of course. Las Vegas has several hostels, and you could also attempt to make a reservation on a service like AirBnB, where the minimum age is 18.
Wherever you choose to stay, and regardless of where you make your reservation, I would call in advance and confirm that they will in fact accept reservations from people under 21. When you check in, the staff will ask for identification, and you do not want to be caught out if they accepted the online reservation in error.