OK, thank you.
I wasn't too clear in the OP. What I meant to say is that the building of developments (of any kind) in forests appear to hinder (delay) their transformation into ancient forests.
I suspect the "development delay" is only my imagination, but I was hoping for a definitive answer whether it is part of the programing or not.
Finally, I appreciate everyone's answers so far. I had not considered the effect of lumbermills on the development of ancient forests, mostly because I don't build that improvement with elves.
PS- Shadowsong, your response was broad enough to address my specific--though ambiguously stated-- concern. I don't want to appear ungrateful or too dull to recognize that. Thank you.