> Older trees that are maintaining a steady state should consume a net of almost 0 CO2.

They are a storage of CO2. If we replaced all old trees with new ones we will gain a lot of potential for CO2 capture but at the cost of releasing massive amounts of CO2 to the atmosphere.

You are right. But I think that the point was to notice that we may have lost many good trees even if we are trying to add new ones.