I am 24

There are many types of mouthguard available from simple soft sports type to carefully manufactured and adjusted ones to treat jaw problems

A correctly made, adjusted and monitored mouthguard should do no harm but a poorly designed one could increase the stress in your mouth and cause you to grind your teeth more often or more aggressively and this could affect your crowns or natural teeth

It will depend a little if the crowns are on your upper or lower teeth! Your mouth guard must have been made for a reason and could be reasonable that it will protect your crowns rather than affecting them. If the crowns are on the lower teeth then they should be protected underneath the mouth guard.

You do not say why the guard was made
In principal there is every need to continue to wear it.
If the crowns are on the upper teeth and the mouth guard was created after the crowns were made then it will prorect the crowns.If the guard was made before the crowns minor adjustments may be needed.