The implications of an age for the open cluster NGC 188 only slightly larger than that of M67 are investigated. It is demonstrated that a differential and absolute comparison of the cluster color-magnitude diagrams leads to no inconsistencies if the reddening for NGC 188 is 0.09 mag larger than that for M67, while the apparent modulus of NGC 188 is 1.8 mag larger. Assuming E(B-V) = 0.03 and (m-M) = 9.7 for M67 results in an age of approximately 5 billion yr for M67 and 6.5 billion yr for NGC 188. The shift required by the differential reddening and distance modulus also removes the Li anomaly recently found by Hobbs and Pilachowski (1988) for NGC 188. If the metallicity of NGC 188 is significantly higher than that of M67, the required reddening differential would be smaller for the same age difference. A younger age for NGC 188 produces no inconsistencies with current theories of stellar and Galactic evolution, and may actually be more consistent than an age similar to the metal-rich globular cluster 47 Tucanae.
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Twarog et al. (1989) studied this question.