What should define the start of the next ice-age?

I’ve often wondered what will be taken as the “start” of the next ice-age. Assuming the start is fairly gradual, we’d expect to see something that either has never happened before starting to happen or something that happens quite often becoming regular.
My own preferred measure would be something very simple, such as the first year in which ice remains continuously on the Scottish mountains.
However looking today I see that WUWT have an article: “4 of the 5 Great Lakes about to freeze over” and I thought that perhaps the first year of a succession when 5 of the 5 might be more likely.
Any thoughts?

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