1.  Marriage Record:

2.  1840 Census Index and Digital Images (online at Ancestry.com; Image #15-16 of 34; extracted by Diana Gale Matthiesen):

1840

PA

Centre Co.

Harris Twp.

pp. 128B-129A

Thomas Johnstonbaugh

010 010 - 000 020 010

0100000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	5-9	1830-1835	= James (b. 1832)
1 male	20-29	1810-1820	= Thomas (b. 1873)
2 females	20-29	1810-1820	= Sarah (b. 1811)
1 female	50-59	1780-1790	= Susanna (b. 1783)
1	person engaged in agriculture

Listed one line above Thomas's brother, Jacob JOHNSTONBAUGH, who is listed seven lines above their brother, George Washington JOHNSTONBAUGH. 

3.  1850 Census Index/Microfilm (online at GenealogyLibrary.com):  Harris Twp., Centre Co., PA, p. 107B-108A, 1349/1371, enumerated 4? Nov 1850 (extracted by Diana Gale Matthiesen):

Thomas Johnsonback	30	M	Farmer
Mary         "	29	F
Alexander    "	6	M
Elmina       "	5	F
Sarah        "	3	F
Julia        "	1	F
Julia Dussler	20	F
[next page]
Joseph Baird	21	M	Laborer		Pa

This enumerator left the birth location blank for everyone on the page, except the first person on the page, with the implication that everyone on the page was born in PA.  Probably a safe assumption.

4.  Census Index:  U.S. Selected Counties, 1860 (Broderbund CD-318):

1860

Johnsonbaugh, Thomas

PA

Centre Co.

Harris Twp.

p. 573

5.  LDS.  Family Search: Internet Genealogy Service:  AF - Ancestral File (online at FamilySearch.org).