We must remember, it was not Kirby speaking, but the song-writer.

Kirby would not, in my opinion, have numbered years he was proud of below their due quantity.

He was more, if he died at ninety-one; and Chillon Switzer John Kirby, born eleven months after the elopement, was, we know, twenty-three years old when the old man gave up the ghost and bequeathed him little besides a law-suit with the Austrian Government, and the care of Carinthia Jane, the second child of this extraordinary union; both children born in wedlock, as you will hear.

Sixty-three, or sixty-seven, near upon seventy, when most men are reaping and stacking their sins with groans and weak knees, Kirby was a match for his juniors, which they discovered.
Captain John Peter Avason Kirby, son of a Lincolnshire squire of an ancient stock, was proud of his blood, and claimed descent from a chief of the Danish rovers.
'"What's rank to me!" cries Kirby; "A titled lass let her be, But unless my plans miscarry, I'll show her when we marry; As brave a pedigree," cries Kirby.' That was the song-writer's answer to the charge that the countess had stooped to a degrading alliance.
John Peter was fourth of a family of seven children, all males, and hard at the bottle early in life: 'for want of proper occupation,' he says in his Memoirs, and applauds his brother Stanson, the clergyman, for being ahead of him in renouncing strong dunks, because he found that he 'cursed better upon water.' Water, however, helped Stanson Kirby to outlive his brothers and inherit the Lincolnshire property, and at the period of the great scandal in London he was palsied, and waited on by his grandson and heir Ralph Thorkill Kirby, the hero of an adventure celebrated in our Law courts and on the English stage; for he took possession of his coachman's wife, and was accused of compassing the death of the husband.

He was not hanged for it, so we are bound to think him not guilty.
The stage-piece is called 'Saturday Night', and it had an astonishing run, but is only remembered now for the song of 'Saturday,' sung by the poor coachman and labourers at the village ale-house before he starts to capture his wife from the clutches of her seducer and meets his fate.