Here we are looking at the deck of the BCP 160 cylinder head that I had installed on a B23F 16 years priorly.

There is evidence of where the coolant was seeping past the head gasket:
...most at cylinder #1 [on the far left],
...some at #2,
...slightly at #3,
...and some at #4.

Age was only a minor player in the WHYs of the head gasket's failure.

The main reason the gasket failed was because the head is a BCP [big coolant passage] head that was installed on a B23.

The BCP heads came out on the B21 motors, that have smaller bore diameters...3.622in/92mm...versus the larger bores...3.779in/96mm...of the B23s.

On the B21s, the 'big' coolant passages are far enough away from the bores so that they are 'behind' the fire rings of the B21 head gasket. This provides a sufficient amount of head metal on top of the fire rings, and HG material behind the fire ring, to maintain a good clamp on the head gasket and maintain the seal.

B23s use head gaskets that have larger diameter fire rings. The larger diameter fire ring actually extends under the big coolant passages at several places on the head. This reduces the amount of head material over the head gasket at those places, and reduces the amount of clamp available to maintain the seal.