The phrase "old as hills" is a common idiom used to describe something or someone that is ancient or has been around for a very long time. There are many synonyms for this phrase, including "ancient as time," "timeless," "antique," "vintage," "prehistoric," "hoary," "matured," "well-seasoned," "historic," "rustic," "dated," and "weathered." Each of these terms can be used to describe objects, places, or people that have a rich history or have a nostalgic feel to them. Using these alternatives to the phrase "old as hills" can add color and depth to your writing or conversation.