It is just an arbitrary cut off point.
They define menopause as having occurred if you have two years of no periods naturally (ie, without having a mirena fitted etc), unless you are over 50 when they stop.
Then you only need one year period free to be declared post meno and more than half of women are over 50 when it happens, hence one year being quoted more often.
It means that if your periods stop on your 49th birthday you will be declared post meno on your 51st birthday.
It also means that if periods stop on your 50th birthday you will be declared post meno on your 51st birthday as well.
Common sense tells me you are in between those two groups, so on your 51st birthday you'll have missed enough periods to be post meno too, unless they come back of course!