Since you can not be in a cabin without someone 21 or older, my cut is that a letter is needed. The 18 yo is considered a minor on the ship.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08
Err on the side of caution, and get a notarized letter, nothing fancy from the parents, all you need the letter to say something like Mr. & Mrs. Jon Smith -with home address give permission to Mr & Mrs. same of home address to take daughter Susan(full name) with them aboard the Freedom of the Seas sailing on this date to this date. The cruise line could ask for this letter or they could not but why risk it. You will also DEFINATELY need a letter of medical consent giving you permission to have medical attention administered to their child if need be, same thing notarized letter with medical insurance name and policy number on it.
I've taken consent forms before that were signed by my ex-wife and the crouise line never even asked for them.
I went on the Disney with my 3 kids and my mother. My dd stayed in the room with my dm and they required me to sign a consent even though the rooms were connecting and I was there. She went a 2nd time and took my neice and they never asked for it. I say better safe than sorry.
Yes take the letter with you. The email says that a signed letter is not needed for 18 and older. I'm thinking there could always be a chance you might need it even though I took one and they never asked.