”Young” here still means ”decades old” (and large, mature trees keep growing thicker fairly consistently which leads to carbon fixation to be a superlinear function of tree age). Plus recent research indicates that old-growth forests actually continue adding biomass quite efficiently. So any planted trees should at least be allowed to grow a lot longer than your standard tree plantations.

Whatever you do with the biomass a hundred years from now doesn’t really matter; either climate change is in control by then or the humanity is screwed.