Over the Hill: Origin and Meaning
This saying dates back to the 1900s in England. It was once commonly believed that during the first 40 years of your life you climbed up a hill and once you reached the top, you were over the hill and the woeful descent began.
This idiomatic expression, therefore means that you are old, past your prime and past your best.
Now, with the 30s being the new 20s and the 40s being the new 30s, the top of the hill mark has somewhat shifted and has lost its value. That said, people still use this expression in a fun-loving or ironic way, when they want to tease someone who has reached an important milestone in life.