Will wearing top and bottom retainers prevent my underbite from coming back? i am done growing(28yrs) if so, how? if not, how can i prevent it again.

Really common: because all teeth have a tendency to relapse, regardless the age, it may be necessary to wear some type of removable retention device at night. Follow your orthodontist advice.

Wear tetainers!: Gums have memory. By wearing your retainers faithfully you will give the gums' muscle horizontal & vertical fibers time to reform and build up around the new placement of the teeth. Not wearing them permits your gums to go back to where they were/what they were used to. You could also ask for a permanent bonded retainer on the back of the lower front teeth. Must keep clean, though!

Wear retainers: At 28 your skeletal growth has ceased, however bone is a living tissue that undergoes remodeling throughout life. Retainers will have little effect on your skeletal (jaw) relationships, but will help prevent dental relapse. The decision as to what type of retainer is best for stabilizing your correction, and how they should be worn, is best made by your Orthodontic Specialist or treating Dentist