Anecdotal evidence suggests to me that European spruce stores more CO2 than it sheds way over 80 years old as a spruce grows in mass significantly decades after reaching that age.
Anecdotal evidence tells me that spruce trees are really good at suppressing undergrowth due to their acidic needless so claiming a mature first is a net emitter makes sense to me. The trees may be growing, but nothing else is.
That’s not true. Methane is a significantly more potent greenhouse gas than CO2, so if the tree is absorbing CO2 but the decomposing needles and leaves are emitting methane, it could easily be a net GHG emitter.
If a tree keeps growing, but also gets more successful at suppressing growth of it's neighbors, then it can be a net emitter overall even though it's a net absorber considered individually.
No, if anything individual trees do start to become emitter as they age due to low growth and decay, how ever in forests old dying trees are quickly replaced by new ones.
However here is a study that tries to bring some light to carbon pools on over-mature 167-213 year-old trees: https://www.mdpi.com/1999-4907/9/7/435/pdf