This is a very complex question. I will try to answer it very briefly.
First of all, the rate of carbon uptake of a forest stand increases with its age. It peaks, depending on which part of the country you're in, between, say, 50 and 150 years of age. On the west coast of British Columbia, that peak could be later. In the foothills of Alberta, it will be earlier.
Basically, the age of a forest determines the rate of carbon uptake, as well as the region where it grows, because that determines the overall growth rate and the maximum amount of biomass that a forest can retain.