When referring to someone who is not quite young but not yet elderly, the term "young old" is often used. However, there are several synonyms that can also be used to describe this age group. One such term is "middle-aged", which typically refers to individuals in their 40s and 50s. Another option is "midlife", which is often used to describe a stage of life characterized by introspection and evaluation. "Prime of life" is another term that can be used to convey the idea of someone who is not yet old but no longer young - it suggests a time of vitality, productivity, and meaningful engagement with the world.