In the 1890s the Scottish scientist Lord Kelvin "proved" that the Earth couldn't be more than 24 million years old according to classical physics ...

Some historians have held that Lord Kelvin's "proof" was a bit tongue-in-cheek, being actually a sop to the theocrats to get an answer acceptable to them, and to get them to quit criticizing things he thought more important. (Note that his "cannot be greater than" doesn't directly contradict the 16,000 yearists, but evades saying that he agrees with them.)

His private correspondence has been reported as implying that he didn't feel he had the needed information for a scientifically valid estimate, and didn't really think such an estimate was very important (at least to his work/interests at the time).

Since none of his private correspondence was sent directly to me, I can't vouch for this argument from first-hand knowledge; but some of the citations give it apparent credibility.

John