I have a Jacuzzi sand filter for my I/g pool - approx age 18 years. Over the last two seasons it has dribbled a bit from the top whenever I change the filter setting (e.g. to backwash, drain, or back to normal pool operation mode). The drips would stop after a few minutes of regular operation. After backwashing yesterday and turning the filter back to regular operation the dribbles became a steady run that didn't stop. I lost approx 1/8" of water overnight, over a 9-hour period. I called a local pool store today and they suggested I might want to try take the head off to see if the filter gasket needs replacing (approx $20). I appreciate the difference in cost between this approach and replacing the filter, but I really don't know much about filter assembly and I'm wondering if it's even worth bothering given the age of the unit. Any comments?

Yes definitely worth a try. The gasket is usually a round O ring and can be easily replaced. When you buy the gasket ask for a small tube of gasket lube too.