Admiral Croft could, indeed, be older than his wife. Four years older, perhaps :) We don't know for sure.

However, witness this exchange from chapter 3, when Sir Walter is generally looking down at the navy. He complains:

"I was to give place to Lord St. Ives, and a certain Admiral Baldwin, the most deplorable looking personage you can imagine; his face the colour of mahogany, rough and rugged to the last degree, all lines and wrinkles, nine grey hairs of a side, and nothing but a dab of powder at top. "In the name of heaven, who is that old fellow?" said I, to a friend of mine who was standing near, (Sir Basil Morley). "Old fellow!" cried Sir Basil, "it is Admiral Baldwin. What do you take his age to be?" "Sixty," said I, "or perhaps sixty-two." "Forty," replied Sir Basil, "forty, and no more." Picture to yourselves my amazement: I shall not easily forget Admiral Baldwin. I never saw quite so wretched an example of what a seafaring life can do; but to a degree, I know it is the same with them all: they are all knocked about, and exposed to every climate, and every weather, till they are not fit to be seen. It is a pity they are not knocked on the head at once, before they reach Admiral Baldwin's age."