Good questions. I don't know. I recall that after DS self-weaned just after having turned 5, a drop could be expressed months later.
It's been awhile for me, DD having completely stopped a bit over 3 years ago. As she got older, except for when she would ask at bad times, like for example when I was in the middle of cooking supper, I pretty much followed her lead as far as nursing went.
She was skipping whole days and eventually a week or more at a time as the years progressed. At 3, nursing was daily, probably at least 2-3 times/day. After she'd turned 5 though, once/day was not even a sure thing. And she only tapered down from there.
I did go through a period getting plugged ducts when she was a toddler, and very uncooperative too she was when it came to trying to get her to nurse in a different, odd, position to work that duct more! I don't know how much of a role stress may have played in that. I'm pretty sure it wasn't a matter of her cutting back so much or too quickly though.
Over all though, as her demand dropped, my production dropped. I'm not sure she was even getting anything every time in those last several months.