Yeah but Dems suck at midterms.
Plus, they can’t change the congressional district boundaries for at least five more years. By then red states will take more seats from the blue ones, because their populations will have grown at a faster rate, and the House of Reps. is fixed to 435 members. Who knows if the Dems will even legally exist by 2022, if they were to pose a threat to Trump in the 2018 midterms?
You’re getting pretty close to doomsaying again, but bear in mind that not much will change in these four years and younger generations tend to vote left.
Trump could just propose a constitutional amendment raising the voting age back to 21 or even older. Congress would pass that proposed amendment in a heartbeat, and I think there are enough republican-leaning states to ratify it before the next midterm.