Binocular Maintenance

I have had two binoculars for about 25 years. An 11x80 University pair of binoculars and a 7x35 Nikon pair of binoculars. Both continue to give clear and sharp views although it seems as though both tend not to hold focus as well as they used to. My question: do people return binoculars for routine maintenance? If so, where would one send them? Is it expensive? Is it worth spending the money for routine cleaning, etc ?? I really like both of these binoculars and expect to hold on to them for years to come. Any suggestions here would be appreciated.