Aged 81 at death in 1888 according to gravestone inscription.

His will is in the Irish Will calendar index:
Record Type Wills Calendars
Date of Death 17 May 1888
Year of Probate 1888
Name Kennedy Boyd
County Londonderry
Residence Killeague

Record Type Graveyard inscription
Date of Death 17 May 1888
Name Kennedy Boyd
Age 81
Parish Aghadowey
Church Aghadowey Church of Ireland
Civil District Coleraine
County Londonderry

From his gravestone as recorded at the Bann Valley site:
57 Boyd-Kennedy-Stewart-Gilmore
In memory of
Kennedy Boyd, died 17 May 1888, aged 81 years
his wife, Mary, died 16 July 1886, aged 71 years
their children
Nancy, died 10 February 1875, aged 29 years
Thos A., died 13 March 1878, aged 31 years
Mary, died 1 August 1864, aged 13 years
Wm. K., died 10 October 1893, aged 37 years
also Jane Stewart, wife of their son, William K. Boyd, died 3 February 1929, aged 75 years
Freddy Boyd, S.O.M.S, a glider pilot regiment, killed at Arnhem, 23 September 1944
Erected by William and Eliza Gilmore, Chase City, Virginia, USA

DoB from gravestone where she is described as aged 71 at death in 1886.

DoD from gravestone as at the BV site.

DoB from LG: John Boyd, son of Kennedy Boyd, born 8 Apr.1836

This is his gravestone in Aghadowey Presbyterian Church:
138 Boyd
Erected to the memory of
Jane Boyd, Cullycapple, who died 29 October 1905, aged 73 years
and her children
Mary, died 31 October 1863, aged 1 year
Alexander Barklie, who died 22 April 1880, aged 10 years
and her husband, John Boyd, who died 17 July 1917, aged 81 years.

From IGI:
Husband John Kennedy Birth: < 1837> <Aghadowey, Londonderry, Ireland Marriage: 31 OCT 1862 Aghadowey, Londonderry,
Father: Hugh Kennedy Mother:    Wife Margaret Anne Boyd  Birth: < 1841> <Aghadowey, Londonderry, Ireland Marriage: 31 OCT 1862 Aghadowey, Londonderry, Father: Kennedy Boyd  Mother:

The DoB for Margaret Anne cannot be right because the Reverend Bryce of Killaig Presbyterian church wrote: Killaig Presbyterian Church shows: Sep 30 1838 after forenoon's service baptised a daughter to Mr. K Boyd, Margaret Ann.
 

DoB from birth record in Killaig Presbyterian Church.

Gravestone says she is aged 29 at death in 1875.

DoB from LG: Nancy Boyd, dau. of Kennedy Boyd, born 30 Sept.1843

DoD from gravestone in Aghadowey CofI churchyard.
Nancy, died 10 February 1875, aged 29 years

DoD from gravestone, where he is described as aged 31 at death in 1878.

DoD from gravestone:
Thos A., died 13 March 1878, aged 31 years

DoB from LG

DoD from LG

DoB from Londonerry Sentinel which gives her as aged 18 years and 9 months at death in August 1864.  On her gravestone, she is described as aged 13 at death in 1864.  One of these sources is incorrect.  I suspect it is the gravestone (or the transcription thereof) as the newspaper one is unusually precise giving months as well as years.

From the Londonderry Sentinel:
Boyd Mary 09-Aug-64 - August 1, Mary, fifth daughter of Mr. Kennedy Boyd, Killeague, county Derry, aged 18 years and 9 months.  Gravestone in Aghadowey Church of Ireland church confirms date of death but gives her age as 13.  Her death record, viewed in Derry FHC confirms her age as 13 and says she died from "chronic heart disease from acute rheumatism after 3 years".  Her father, Kennedy Boyd was Present at Death. 

DoB from LG - Catherine Boyd, dau. of Kennedy Boyd, born 8 Oct.1853.

DoB from gravestone where he is described as aged 37 at death in 1893.  Date and month from LG.

DoD from gravestone:
Wm. K., died 10 October 1893, aged 37 years

DoB from LG