Can't check-in to a hotel because I am 18

Stay at a hostel. You shouldn't have any trouble due to your young age. You will have to buy a hostel membership, either an annual or temporary one. But the nightly fee will probably be less expensive than most hotels anyway.

There is one in Syracuse. Here's the link to check availability: http://www.hiusa.org/new-york/syracuse/downing-international-hostel

You will need to search hotels individually for the most part and mostly away from chain hotels like Choice Hotels, or Wyndham and the like.

For example Motel 6 policy is to allow guests of 18 years of age and older to check-in but individual hotels may deny until 19 or 21.

Hotels.com lists Syracuse Inn & Suites. Hilton provides Hilton Garden Inn, Expedia gives Ramada and so on but before going there I would suggest contacting them to verify that information is actually true.

According to Expedia.ca (unfortunately you can't search by that parameter)

Ramada Inn Syracuse Buckley Rd North Syracuse NY 1-855-247-4371

There may be others... apparently it's all over the place from 18 to 25 depending on the hotel.

Assuming you truly are a responsible young adult…

Contact the hotel manager directly, well in advance. Explain your situation. Demonstrate that you are a responsible young adult who is not going to wreck the place with a wild party or other crazy behavior. Be polite and courteous. Put yourself in the shoes of the man or woman responsible for the care and upkeep of the building and business.

Hotels don't hate young adults. They hate wild, drunk, out-of-control young adults. Unfortunately, a generation of bad parenting has made this all too common.

If you succeed in getting their approval, ask the manager to leave a note on your reservation records to avoid problems if the manager is not present when you check-in. Ask them to email you a note as well.

If you are a hooligan looking to use this approach as a scam, remember that not only can you be evicted from the room, but you can also be arrested for causing a disturbance or damaging property.