Is there any possibility she suspects you of having an affair/ONS at the time when she started going cold?
Has she asked you in the past to pick up more of the housework/childcare/household admin and you've temporarily stepped up but then got back to old habits? I don't mean to accuse you of being a lazy beggar but it's such a familiar story on these boards, and it's a common relationship ender after a time because it slowly erodes the love and respect felt towards the one who isn't pulling their weight.
The timeline would fit that scenario - she's basically waited until the eldest is through their A-levels (I assume) and has spent the run up emotionally detaching from the marriage.
It means she is 6 months or so ahead of you in terms of preparing for separation and mourning the end of the marriage.
Another man is certainly a possibility, or maybe she has just had a simple flirtation and thought "Hey I've still got it, I don't really fancy my H any more - the kids are old enough now to look after themselves if I want to go on dates."
I think unless she's mentioned physical symptoms associated with menopause, such as her periods stopping (or getting much heavier), hot flushes, night sweats, that is probably a red herring I'm afraid.
I'm 46 and entering menopause and I have all of the above and the most ridiculous emotional over-reactions and mood swings. I don't have a partner but I can't imagine being "inspired" to break up a marriage of that length by my mood swings.
How have the DC taken it? Have you been able to make regular contact with them?