for your wife's children she needs to prove she has sole custody and sole responsibility.
your wife can apply anytime to bring them here, she could have applied to bring them on her fiancee visa.
no don't wait til he's 18 and a student, he might not get a student visa, it will be very expensive - will have to pay international fees and he probably have no right to stay in the UK once his studies have finished.
get them here asap, or as soon as your settled, it will be cheaper and easier.
