1.  Charles Frederic Farlow.  1911.  Ballard Genealogy.  Charles H. Pope, Boston, MA (available as a facsimile reprint from New England Historic Genealogical Society, Boston).  Says they were married 26 Apr 1682.  Will proved 15 Jan 1723/4 (Plainfield Probate Records).

2.  Lucius Manlius Boltwood.  1849.  "Marriages in the Town of Andover, MS., from 1647 to 1700."  New England Historical and Genealogical Register 3(Jan): 65-68.  On p. 67:

William Ballard & Hannah Hooper married Apr 20 1682

3.  George M. Bodge.  1889.  "Soldiers in King Philip's War."  New England Historical and Genealogical Register 43(Jul):  259-279.  On p. 261, among "items of credit" in the "Treasurer's book":

AT THE GARRISON AT CHELMSFORD... January 25, 1675-6... William Ballard  02 08 00

Is this our subject?  Or  some other William Ballard?  (See next source.)

4.  Anon.  1902.  General Register of the Society of Colonial Wars, 1899-1902.  Publ. by Society, NY (online at GenealogyLibrary.com):

p. 545	A. & H. A. Co., Abbreviation for Member Ancient and Honorable Artillery Company of Boston.
p. 556	BALLARD, WILLIAM, 1603-1689.  In King Philip's War in garrison at Chelmsford.  A. & H. A. Co., 1636.

Here we have William BALLARD of Lynn (1602/3-1638)  mixed up with William BALLARD of Andover (c1616-1689).   King Philip's War was fought 1675-7, at which time William of Lynn was dead and William of Andover was nearly 60 year old.  It seems far more likely to me that the garrisoned soldier was William BALLARD, Jr., son of William of Andover, or some other younger William BALLARD.