I tend to agree with the two previous posters. In my current research I have someone who took 10 years off his age on the marriage certificate. He said he was 69, when in fact he was 79. Then again, his bride was 45 at the time, so maybe he had a good reason for understating his age. Interestingly, he may have also understated it in the 1939 Register. As an aside, it's quite a difficult one:
The subject's true age in 1939 was 77, but does this look like 67?