At 5'5" and 240...with good luck, your skin will snap back. At age 18 your chances are terrific (but not guaranteed.) I think about the tiny little 20 year olds I know with huge pregnancies whose skin reduces and looks terrific in months.
I had my surgery done at 46, and was stunned at how well things reduced back into place. I have a few trouble areas, but I don't mind how I look naked at all. I was open to skin surgery, but don't feel I need it.
At 18, I think you're gonna look amazing:) Best wishes.