Although old trees contain huge amounts of carbon, their rate of sequestration has slowed to a near halt. A young tree, although it contains little fixed carbon, pulls CO2 from the atmosphere at a much faster rate.
***Something I didn’t know before. Now I have to verify it because I can’t trust the source.
On its face, that seems perfectly reasonable.
<speculation>
Consider the growth rates of older trees versus younger ones. As a tree matures, its growth rate slows. A younger tree, growing more rapidly, grows by adding wood to its height and girth (more than than an older tree does). That wood is largely made up of carbon which it has removed (as CO2) from the air.
</speculation>
IANAA. (I am not an arborist.)