@La***
, to answer your question, that over 50 being taken care of by children who are over 20 extension came about 3 years ago when they re-vamped the immigration extension rules.
Personally I have never seen someone get it, although in reading it the requirements seem straight forward.
When it first came out, I showed the thai version to the immigration officer and pointed out that it did not specify any financial requirements at all. BUT she said it's 800K baht.
I don't know honestly, but it sure ain't written where it says you need to bank any money to get the extension to be under the care of your adult children.