You need to have 15 days remaining at most immigration offices (21 days in Chiang Mai) with your current stamp on the day of your application for in-country Non-O based on being over 50.
You need to go to your local immigration office to get the latest handout (immigration office specific) of requirements.
Here is the general requirements list as FYI.
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/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf