A lot of the older population in ND are old-school, farming, WWC Dems. The younger generation is Likely R due to the oil boom.
Nationwide polls suggest that elderly voters (65+) are turning against Trump. Maybe they have memories of Richard Nixon's dirty tricks and best recognize Donald Trump as monstrously abnormal in American politics. They have never seen his antics in American politics, and they find his conduct terribly upsetting.
Note also that farm owners are a very old demographic, and Trump's trade war hurt them. North Dakota farmers stand to be hurt by the tariffs and trade war that cuts commodity prices (revenue) and increases costs (anything imported or that can be consigned to pay for something imported, as with vehicle insurance... in case your auto liability ends up paying for foreign-made parts for a vehicle). Farmers buy much imported stuff, and tariffs are in essence a steep sales tax on imports, even if the tariffs have some exemptions.
But with farm owners are also farm families... and those are chips off the old block in politics. If you are 40 and still living on the farm where your 68-year-old parents are, you want the family farm to prosper.. and you know what butters your bread (if you are a dairy farmer) and where your bread comes from (if you are a grain farmer).
The younger generation in North Dakota may be more conservative due to the oil boom in North Dakota, as Trump is extremely sympathetic to the energy industry -- even to the extent of trying to get people to buy gas-guzzling vehicles as replacement for gas-sipping vehicles. But this said, those oil-field workers buy imported stuff, and when their i-devices start getting much more expensive due to tariffs on imports they won't be happy.
Tariffs are taxes -- bad taxes, capriciously imposed and administered. tariffs can more than offset the minuscule scraps that people other than the super-rich get in cuts in income taxes. I doubt that that goes well in even the most conservative states.
The AARP poll demonstrates that older people in North Dakota, as in Maine (a very different state in demographics and political heritage), are no longer sympathetic to Trump. Note well that the average age of a voter in America has typically been over 50. This age group is still amenable to mainstream GOP values. I'm guessing that North Dakota politics heavily involve farm families and their interests, and that so long as Republican policies do not hurt farm interests, farm families largely vote Republican out of concern for taxes... in most political years. 2016 may have seemed normal enough in North Dakota, but 2018 isn't, and 2020 probably won't be.
North Dakota has not voted for a Democratic nominee for President since 1964, and hasn't otherwise voted for the Democratic nominee for President since 1936. It was one of only twelve states to vote for Dewey in 1944 and one of only ten to vote for Willkie in 1940. In 1928 it may have been more Democratic than the USA as a whole in Hoover's blowout defeat of Al Smith in 1928 (roughly 55-44, nationally 58-41 for Hoover -- economic distress was hitting farm states before the 1929 Market Crash), but it went nearly 70-28 for FDR in 1932.
Wild swings can happen, and I am not predicting one in North Dakota. Farmers and farm families vote their pocket-books, and they are not stupid.