John Houston (15)'s DOB is commonly given as between 1685 and 1690. The basis for this is not usually stated. The following provides an fact based assessment of his likely DOB.
John Houston, my grandfather, came from Ireland with his family when my father was about nine years old, about 1735, bringing with him his mother and wife, who was a Miss Cunningham, and all of his children, excepting the oldest son, James, who died soon after the family left him.
The above passage holds that John Houston (15)
Accepting these data as factual John (15) could have been born no later than 1698.
The commonly cited DOB's for John (15) lie between 1685 and 1690. While considerably earlier than the maximum year of birth of 1698, these dates are not inconsistent with the available data. It is possible that John (15) married considerably later in life than 1716. An age at marriage of 27 to 31 years of ages would not be unreasonable, and would be consistent with the commonly cited DOB's. Alternatively, the eldest son may have been born in 1716, but may not have been the eldest child. Available data on the childrens DOB's does not suggest that there were in fact older children in this family.