Depends on the forest. It's that simple, some mature forests, are great carbon reservoirs, but their ability to bring additional carbon in, is very limited. Other old growth forest continue to become more complex systems that can store more carbon.

All the research I went through as a grad student pointed to younger forests being better at sequestration. The bottom line is: the best carbon sinks are forests that don't exist yet. Allowing a field to go to forest is going to store more carbon over 100 years than any other option.