When to replace a tire when tread is good?

The mysterious 'frank121 said:

I have some tires mounted that I have ridden the past couple of years
(several bikes, not many miles past two years) that look good as far as
tread.

I am wondering what to look for as far as the tires getting dry around the
area where they hook into the rim. Can this dry out and cause problems and,
if so, how can I tell?

If the tire isn't lumpy or misshapen, it's structurally sound.

If it doesn't have excessive tread wear, there's no reason to replace it.

Age, per se, doesn't cause a problem with modern tires.

Sheldon "Don't Borrow Trouble" Brown
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