Trees supposedly take 30 years *before* they absorb CO₂. Why? - Feddit

I often hear science-adjacent folks stating that a tree needs to be 30 years old before it starts absorbing CO₂, usually paired with the statement that it’s therefore pointless to start planting tons of trees now for slowing climate change. Now, as far as my understanding goes, the former statement is very obviously nonsense. As soon as a tree does photosynthesis, it takes carbon out of the air, which it uses to construct cellulose, which is what wood is made of. Really, it seems like it would absorb most CO₂ during its initial growth. I understand that it needs to not be hacked down + burnt, for it to actually store the carbon. But that would still mean, we can plant trees now and not-hack-them-down later. I also understand that some CO₂ invest may be necessary for actually planting the trees, but it would surprise me, if this takes 30 years to reclaim. So, where does this number come from and is it being interpreted correctly? Or am I missing something?