18 year olds are kind of too cool for school so maybe try band or singer style poses... moody leaning against a wall, nothing silly or over posed. Try them not looking at the camera and in to negative space, or looking directly at the camera but not smiling.
14 year old will be more tricky... do they play the guitar or football? Both would be good as natural and as they'll no doubt be an awkward teenager (they all are I think) means again no real posing required. Playing the guitar is easy to get good shots of... doing keepy ups with the ball is a good one if they play football
Just my thoughts but hope they are helpful