"Over the Hill" in this case, means that you are no longer young. You may be old but you are not a young person anymore.

If you live in a country like America then you might celebrate this event with a party!

When I became 40 years old my family and friends had a party for me. It was an "Over the Hill" party.

The colors were black and gray, to signify old age and death. The food was all soft and boring, something an old person without teeth would eat. The gifts were also something appropriate for an old person. False teeth, a wig, make up, skin lotion, some small exercise equipment, a girdle, things like that.

The party was done for fun and in a funny way so everyone had a good time. This is how an "Over the Hill" party is done. Use your imagination, create a unique party and surprise your (old) friends with their very own "Over the Hill" party.