My wife asked about this at the Holiday Inn front desk when we were at CP a couple of weeks ago, as our son is turning twenty, and he's looking to get "the gang" together for a trip, sans-parents.
The front desk employee claimed the HI would, in fact, rent to those over 18, but under 21, but would require a damage deposit at sign-in.
I think the attitude towards under-21 guests often reflects the area. Sandusky and CP want to highlight family values, so many hotels would rather not have teenagers on their own in their establishments. College towns, however, seem to be far more open to renting hotel rooms to younger people, as that group is a huge part of their business.
Best Advice: Phone First!