I'm 20 yrs old and when I smile or laugh my mouth opens very wide. Is it possible to make my smile not as wide?

Serously, this one does not make that much sense.  No there is no surgery for this.  If you spend more than a few moments thinking about this you might also ask yourself if your concern is reasonable or if you might have body dysmorphic disorder.  If you spend hours a day thinking about a concern that is not obvious to others or does not exist,  or stay away from social events because you are concerned that others are talking about this issue, or making fun of your appearance, this is not normal.  If this is a hypothetical type question, then here is your answer.

This is a significantly delicate and tricky proposed concern.  Overall there is not a recommended surgical procedure.  However, based on your anatomy there is a possibility that some expertly placed small degree of Botox can diminish some of the widening of the smile.  However, nonetheless this has to be done, performed and assessed by a physician who does a significant amount of Botox treatments.  Overall, I think that it is important to get a good opinion from a physician as to whether anything, if at all, should be done.  Try to trust the opinion of the physician that you consult with and if you have a trustworthy physician and a physician who is well recommended and if this physician says don’t have any procedures done then importantly you should listen to this recommendation.  

A big wide smile is beautiful!  To adjust the appearance of a smile more than likely would require incisions on or around the mouth which may lead to an unattractive scar.