UPDATE:
Because someone questioned Lady Wentworth's age on another forum, I asked a correspondent, Michael Andrews-Reading, who has access to many old records, to look into this.
His reply is this: "The marriage licence dated 9 February 1666/7 for William Wentworth, aged thirty, and Isabella, daughter of Sir Allen Apsley, states she was then aged 18, so born circa 1648."
The date "9 February 1666/7", in modern terms, is 9 February 1667 [the new year didn't used to begin until 25 March].
Thus, on 9 February 1667 Isabella was 18, giving her a birth date in 1648 or 1649. She was thus 57 or 58 in February 1706, when she wrote about her great-grandchild, which presumably had been born not too long before, probably in February 1706, though possibly a month or two before that.
Having a great-grandchild at the age of 57 or 58 is not as unusual as having one at 53, of course. It should be remembered that Queen Victoria was twelve days short of the age of 60 when she had her first one.