..one short answer to one question somehow begets yet more questions... in this case two more!

I can't recall the exact ages from NA, but wouldn't that have meant an awfully long engagement?

I can't remember either, offhand. So maybe those interested can bounce back to the NA board to work out what their ages actually are.

Was in common for couples to be engaged for years?

Big question: in a population of about ten million, how many couples would count as "common"? How many engagements would be officially recorded in some way so that we could count the number of months between engagement and marriage? (answer- not many, I think).

All you can do is look at a) Jane Austen's characters , b) Jane Austen's family, and c) Similar gothic novels of the period. And what do we find?

a) Characters- Edward Ferrars (S&S)& Frank Churchill (Emma) both in the position of having long (and secret) engagements. James Morland? Possibly. But nobody, I think, in P&P, MP or Persuasion.

b) In Jane's family- Jane's sister Cassandra had to wait to get married for several years. At at least two of her brothers only got married when they were financially stable enough to do so, rather like James Morland. I'm not sure how long either of them were engaged before the marriage actually happened. (For reference, I would look to one of the many biographies of Jane Austen, but since I don't have them at hand right now, I have to rely on memory.)

c) Offhand, I can't remember any gothic novel that uses a long engagement as a plot device. However, somebody else might like to contribute to this idea.

Which is a very unscientific way of saying that I would guess that long engagements were not particularly rare, and that it's more a commentary on the realities of Jane Austen's world than it is on the literary aspects of the book.