That's true, but they don't fix any more carbon after they're fully grown. I'm not against tree planting, but it's probably a bit oversold as a solution. Kelp probably presents a more interesting opportunity.
I know at least conifer trees are never "fully grown". I imagine it is the same for deciduous trees. Trees continue to add wood to there structure every year. You count the tree rings to know its age. A 500 year old redwood, cedar, or Douglas fir can be 15 or more feet in diameter. That is a lot of wood. Unlike what many people thought, studies have found that the volume of wood added to trees in a mature forest is similar to a young one.