"Benjamin (I: 439) had an eldest son, Isaac, proved by Stratford Deeds, 3: 93. Doubtless he was Isaac of Derby (see record, I: 438), who d. 12 Apr. 1733 in 49 yr. (g.s.), but the age may be overstated as he did not marry until 1723, and his birth abt. 1684 would make his father Benjamin (b. 2 Feb. 1665/6) marry very young."
On the other hand, Isaac's wife, the widow of Abel Gunn of Derby, was born in 1681 which already makes her three years older than Isaac's age as calculated from his gravestone. This is not an impossible difference in ages, but is somewhat unusual for the time.