OK, this is a very simple idiom, although I’m a bit confused about its meaning. I found many explanations, from one that says it means ‘old’ to the one that says its meaning is ‘full of acne’.

Can anyone tell me the definite answer?

"Full of acne"!?

I’ve never heard that.  As far as I know, "the hill" is age 40 specifically, or unspecifically, just middle age.

So "over the hill" in reference to a person’s age means simply over 40.

If the term is used to refer to the age of something other than a person, it just means old and/or connotes deterioration or that the thing is outdated.  For example, I might refer to my car as "over the hill;" the thing is not 40 years old but in terms of car life, it’s old (a 1980 model).  I also have a 1964 car, and it will be over the hill in both senses next year!

inanna

From the AHD entry for hill:

Idiom:
over the hill Informal
Past one’s prime (see noun def. 3 and 4).

So an athlete could be over the hill in his thirties, or even younger in some sports.

I’ve never heard the acne reference before, either.  

Past one’s prime is an excellent definition. The prime of a tennis player isn’t the same thing as the prime of a professional stamp collector.

While many people seem to find 40 the definition of the apex of the hill in question, I believe this is changing. There are so many baby boomers now, whose apex was reached 15 years ago, but who refuse to acknowledge even the thought of a hill, much less that they might be over it, that the 40 shifts to 50, then to 60. After all, aren’t we baby boomers going to live forever?  ;)

- PW (working on his second hill)

40 is just an arbitrary number. If you want a more definitive point it would be "someone older than me."

On the serious side, when someone gets to the point where they are no longer learning, developing new skills or talents, or creating, they start declining. Noone it seems, spends much time at the top on the hill.