From Westmoreland County, Virginia, Order Book, 1675/76-1688/89, Part 1, by John Frederick Dorman: "Page 120, 17 April 1678. Lt. Wm. Spence is exempted from paying the publick levy, being above 63 years of age." If William was over 63 years old in 1678, that puts his birth year at 1615 or earlier, so there was at least a 19-year age difference between him and Patrick, maybe more.
I have that William and Eleanor also had a son named Dr. Alexander Spence, who married Elizabeth Brown. This Dr. Alexander Spence is not to be confused with Patrick's son Alexander. Both Alexanders appear in several of the same records, with William's son always identified as "Dr.," and Patrick's son identified as "Gent." or "Capt."