article that shows you different types of standards as it relates to age, and booking a hotel room. I'll summarize it. Basically, the legal age is 18 for booking a hotel. Depending on the region, area, state, county, or place, they can set their own standards of the legal age. For example, a county sets their age at 20, so, all of their hotels in that county requires the person to be 20.
Another factor is the type of hotel. That is, a low budget, mid budget, or high budget hotel. Most low budget hotels requires you to be 18, some mid budget requires you be 21, and some high budget requires you to be 25. The hotels that require a person to be 25 want to attract a different demographics than those requiring an 18 year old.