I hope someone will correct us if we're wrong, but that was also my understanding, that Sir William Thornhill "has scarce attained the age of thirty..." This is related by Mr. Burchell, who is also described as "a gentleman who seemed to be about thirty..." Hmmm... I don't find the age of Mr. Thornhill (the nephew), but am guessing he is at least age 18 or even in his 20s.