Presumably you would have built up a pension in your previous country, which you can draw from. That would satisfy the requirement for an independent source of income. When Britain was in the EU it was (and still is) common for pensioners to live in Spain.

The comment you're responding to is wrong. Yes of course you can stay there, you can even move there post-retirement. You just can't get the very lowest level of benefits unless you've lived there for long enough.

You gain the right to permanent residence after 5 years of residence under the other conditions.

The 65 year old would need to live from their pension for a couple more years.