Is 6 hours of sleep enough for an 18 year old?

No. 6 hours of sleep is not healthy for the average 18 year old. At this age up to 8-9 hours are needed. If you are asking the question, i think there might be a concern. I would see a sleep specialist just for an initial evaluation.
Can be. Not everyone needs the beauty sleep of 8 hours....This is average....Which means many need less than 8. Some people can do 2-3 hours/day and are fine for many years...I actually have 2 patients needing only about 3 hours or so... Some need 9-10 or more... As long as you don't feel tired and don't have the need to "catch-up" on sleep, functioning normally, then i don't see a problem....
Doubt it. Most in that age group are involved in training/school/extra curricular activities, etc.A basic 8-9 hours are required at that age with increases needed if studying hard or playing sports. Trying to get by on less leads to subtle declines in memory retention, performance,resistance to illness etc. Some will sleep walk/talk. All of these improve with better sleep habits.