The marriage record for George Douglas and Janet Gunn lists her as 
being from Halkirk.  George and Janet were married in 1811 so it may be 
reasonable to expect her age at marriage to be somewhere between 14 and 
24. From the IGI, there are 5 Janet Gunn's with recorded Christenings 
from Halkirk between 1787-1797:

 Jannet GUNN - C 29 MAY 1790 Halkirk
	Father: Benjamin GUNN	Submitted 7431731
	Mother: Henrietta

 Janet GUNN - C 07 JAN 1791 Halkirk
	Father: Donald GUNN	Parish C110374
	Mother: Elizabeth MACKAY

 Janet GUNN - C 25 DEC 1791 Halkirk
	Father: Alexander GUNN	Parish C110374
	Mother: Elizabeth GUNN

 Janet GUNN - C 26 APR 1792 Halkirk
	Father: John GUNN	Submitted 7431731
	Mother: Margaret MOWAT

 Janet GUNN - C 17 DEC 1792 Halkirk
	Father: John GUNN	Submitted 7431731
	Mother: Jean ELDER

Of these, I picked the last Janet GUNN because she was the 
first I found on the microfilm:

 Gunn   Janet lawful daughter of John Gunn and Jean Elder
        in Halkirk was baptized 17 December, Donald Elder and
        Henrietta Buchanan there Witnessed

This would make her 20 years old when she was married.

This could use further investigation.  Perhaps her date of birth could 
be found on her tombstone.