The aircraft can handle it, but can the Y passengers. The smells from the lavs, meal trash and passenger B.O. will be overwelming after 18 hours.
In my younger years I flew in the back of a Boeing 707 for many 18 hour flights (in-air refuled) and it was always a wear on the body. I now limit my flights to 6 hours in Y or 14 hours in J. If travel requires longer flights, I work in a stopover.