First of all, I know nothing about genealogy. My mother was a Doolittle, and she inherited some interest in the subject from an uncle, who wrote a bunch of little volumes called, "The Doolittle Family in America", which have descended on me.
Here is what I found with a few minutes' research.
According to "The Doolittle Family in America" (Volume I, page 103), Timothy Doolittle was born in Middletown, CT, February 17, 1752. He was the son of Nathaniel Doolittle and Hannah Foster Doolittle, who settled in Middletown, the birthplace of Nathaniel's first wife, Mary Cornwall. According to this source, Timothy served "among the N.Y. troops in the Rev. serving under Col. Van Ness, also Col. Graham."
Again, according to "The Doolittle Family in America" (Volume II, p. 190-1) Timothy had one son, "Isaac, prob. d. age 93".
I have found no record of the long-lived Isaac, nor of any of his offspring.
I don't know if you have already done it, but you might contact the genealogist of the Doolittles of America, Ed Doolittle, at email@example.com.