Hotels in Maine that allow 18-year-olds
If you are 18 and planning a Maine trip, sort out the hotel before anything else, because some hotels there set the check-in age at 21. Maine's largest city is on this page because it has 18+ hotels. Elsewhere in Maine you can still hit a hotel that will not check you in under 21, but that is not a reason to give up on Maine.
2 hotels
Portland
Maine's largest city shares a name with a city out west
That city is Portland, Maine's largest, and the shared name is where the mix-up starts. Type Portland into a flight search without adding the state and you can end up pricing flights to the Portland in Oregon, on the opposite coast, while your hotel is booked in Maine.
The airport code is how you keep the two straight. The airport in Portland, Maine, is the Portland International Jetport, code PWM, while PDX is the code for the airport of the Portland in Oregon. If you are booking a hotel at 18 and a flight to go with it, the flight and the hotel need to be on the same coast, so search PWM, not PDX.
The Jetport is about five miles from downtown
If you fly into the Portland International Jetport, downtown Portland is about five miles away, which the airport itself calls about a 15 minute drive. So a downtown hotel is close to where you land, not a long trip from the airport.
If you are driving rather than flying, Portland sits just off I-95 in southern Maine, so you are not going deep into the state to get there.