Example:
Iain: “Hi, Ligia! Happy birthday!”
Ligia: “Thank you, Iain! I am so happy it’s my birthday!”
Iain: “You are turning 30, right?”
Ligia: “ I am actually turning  37… Pretty soon I’ll be over the hill.”
Iain: “Ligia… you are still young! You’ll not be over the hill anytime soon!!”
Ligia: “You are right! I’ll say I’m over the hill when I turn 85! Haha.”
Iain: “That’s better! Enjoy your birthday!”

Meaning: Over the hill means to be too old!

Happy Birthday Ligia!!!