Over the hill is a phrase that is used to describe a person who is no longer young or who is past their prime. There are a few different synonyms that can be used to describe someone who is over the hill. One option is to say that someone is past their prime. Another is to say that they are past their peak. Some other synonyms include aged, ancient, senior, elderly, mature, and old. All of these words can be used to describe someone who is no longer young, but they each have their own nuances and connotations.