I declined G+ for a while, but eventually pressed a button and consented to convert my accounts. I am not aware of any automatic conversion, especially from 10-year-old accounts. I assure you, my Gmail account is older than that.
Either it stops being optional after some point, or they tricked me into converting vis-a-vis some Dark Pattern (which is if anything even more scummy as far as I'm concerned). My YouTube account is separate from my Gmail account and I can assure you that I never would have added a g+ page to it on purpose given the choice.
