That's the one that counts, really, because you'd want to know (as a married person, that is) how many other marriages are generally failing around or not. When you're married and you feel that your marriage is in a rough spot you're not really interested about any statistics involving people that haven't reached the legal age to marriage yet or about 70-year-old widows, you're interested about people around your age who are in the same legal situation as your are, i.e. are married.

And yeah, I've seen the wikipedia explanation that says that "this measurement compares two unlike populations, those who can marry and those who can divorce" but afaik you kind of know if you're going to divorce a person in the first 10-15 maximum 20 years of your marriage, and 10-15 even 20 years is not that big of a timeframe for a major imbalance to form between the number of "people who can marry and and those who can divorce". In other words populations in Western countries kind of remain "stable" for a given period of 10-15-20 years, so it's not like suddenly we've got a lot more old people compared to young people so that the divorce/marriage ratio would become skewed. The old vs young imbalance takes a lot of time to build up (I'd say 30 to 50 years) or a catastrophic event like an world war.