Reaching the average mid-point in life, which is age 40. Therefore 40th birthdays are generally thought of as making it "over the hill". You've gone up the hill for 40 years ⬆️ (healthy, youthful appearance, etc.), now 40 more years down the hill ⬇️ (decreasing health, loss of physical beauty, etc.)
My mom is gonna be over the hill this month, she's turning 40!
In the rest of the world over the hill refers to someone over 40 or 50 but in Birkenhead your considered past it if you aren't married with kids at 19.
Person 1: OMG see that girl there? She's well over the hill her, shes 22 AND aint got no kid or even been married.
person 2: OMFG old hag!!!!