im not a dental professional but i know a little bit about broken teeth and trying to fix the. do you mean crowns? maybe it has been fixed with composite resin or something, which doesnt really have a long life and will probably fall off sooner or later, and they are suggesting a ceramic crown which in theory will last much much longer. maybe they suggested it now because you are fully grown, or, surprise surprise, since you are now 18, they can charge you a lot of money for treatment, which is my theory about greedy money grabbing dentists. maybe if you are having no problems at the moment you should just stick with what you have and save up the cash to get the best long term solution (which if i remember rightly is a ceramic crown), and just wait till the current one falls off. i dont think there is any particular benefit to fixing what's not broken at the moment, apart from it might look a bit better if its a front?