Your EPA document is about sequestration rates PER TREE in (sub)urban setting. Under such circumstances, the CO2 sequestration rate would depend mostly on the amount of sunlight that it can capture, which increases as the tree grows. A typical urban tree looks like a short stick with a strongly branched green ball on top.
In a production forest, trees are planted closely together and compete with each other for light. You'd expect the photosynthesis rate per unit of ground area to level off once a full leaf coverage is reached, which could well happen within 20 years. My idea of a production forest is lots of tall stems with little branching, most leaves near the top, and not enough light to support plant growth at ground level.
Secondary effects could be that the trees waste more energy on forming new leaves every spring that capture a smaller and smaller fraction of the sunlight as the competition for light increases. That could well lead to a reduction in net sequestration rate over time.
To me it seems plausible that you're both correct, but comparing apples and oranges.