@Gregor *********
The OP is asking about extensions. A 1 year extension can be based on retirement or marriage. If your original visa was an O-A then you need insurance for an extension based on retirement but not for an extension based on marriage (or so the reports say).
You are correct that the visa, obtained outside of Thailand, is solely for those over 50 years old and they require insurance.
A visa is not an extension. It is the extension that the OP is asking about.