...Before caving completely to Fanny?

] I think that's exactly what he means--that she could not be expected to live until age 55! or even to age 50 if he literally means ' half that purchase'

] This is perhaps not surprising for a teenager to see 55 as being very ancient, but John Dashwood has to be at least 26 years old, and you would think his perspective would be different.

I'm guessing it is different - He must realize that Mrs. Dashwood's life is quite likely to extend beyond fifty. But John was the one who first suggested the annuity idea, which Fanny has just shot down with "if she should live longer than fifteen years). I think this is John's last feeble stab at keeping the annuity idea going, even with such a laughably untrue statement as to say that Mrs. Dashwood will probably die in seven years or less. He doesn't believe it, but it's a way of keeping the idea in the air. He did make that deathbed promise, after all, and it's probably still pricking him slightly.

Granted, Fanny makes short work of him and his pricks of conscience. But even though John isn't the swiftest of men, I don't think he literally believed what he was saying - he was just trying to keep Fanny's rapacity at bay for a few more seconds and failed dismally.