If the tree is healthy, the fruit will just get better and better the older the tree becomes. At this time of year there's what are called historical groves with citrus trees that are 100+ years of age. Their fruit sells at a premium price, due to their exceptionally excellent quality and taste. Dr. Malcolm Manners, who almost everyone on this form knows, once wrote on the old forum, that the very best fruit he has eaten was a March seedless grapefruit taken from a 25 year old tree.