We have 18-year-old Holly bushes (about 10 ft. long, 4 ft. wide, and 4 ft. tall) in our front yard and they seem very healthy. We had two spruce trees on each end of these bushes. We have decided to replace the spruce trees with new Holly bushes. My question is, should we pull out our 18-year-old holly bushes and replace them with new ones or are there still a few years left in their life span? How long they are supposed to live?