Given the age difference between Samuel and Leah, it's possible he had a prior wife.

1.  Marriage Record(s):

2.  1840 Census Index/Images (online at Ancestry.com):  not found.

3.  1850 Census Index/Images (online at Ancestry.com; Image #12 of 14):  Greenwood Twp., Juniata Co., PA, Roll 786, p. 307B/614, 1983/2012, enumerated 1 Nov 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):¤

Samuel Dimm	45	M		Farmer	4500	Penna
Leah Dimm	27	F				"
Joannah Dimm	6	F				"
Jane Dimm	3	F				"
Wilson Dimm	1	M				"
Catharine Shallenbarger	24	F				"
John Grimes	17	M		Farmer		"

Catharine is probably Leah's sister.  Listed next to Jacob DIMM.

4.  1860 Census Images (online at Ancestry.com; Image #150 of 430):  Millerstown P.O., Greenwood Twp., Juniata Co., PA, Roll 1119, p. 622, PN 216, 1497/1, enumerated 31 Jul 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):¤

Saml Dimm	54	M		Farmer	9000	2000	Penna
Leah Dimm	39	F					"
Martha J	13	F					"
Wilson	11	M					"
Franklin	9	M					"
Irwin	5	M					"
Thomas B	3	M					"
John Graham	24	M		F laborer			"
John Drawburgh	60	M		F Lar			"
Mary Stroup	17	F		domestic			"

Mary STROUP is Samuel's niece, whose mother, Mary (DIMM) STROUP, died in 1847. 

5.  1870 Census Images (online at Ancestry.com; Image #2 of 19):  Richfield P.O., Greenwood Twp., Juniata Co., PA, Roll 1353, p. 378B, PN 2, 9/9, enumerated 22 Aug 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤

Dimm Samuel Sr	62	M	W	Farmer	18,000	1100	Pa
____ Leah	48	F	W				Pa
____ Martha	2?	F	W	at home			Pa
____ Wilson	21	M	W	Carpenter			Pa
____ Frank	18	M	W	????			Pa
____ Jerome	15	M	W	"			Pa
____ Thomas	14	M	W				Pa
____ Simon	10	M	W				Pa
Stroup Mary	25	F	W	House Servant			Pa

6.  A.L. Guss.  1886.  "Chapter XXI: Greenwood Township." Pages 885-891 in F. Ellis & A.N. Hungerford, eds.  History of That Part of the Susquehanna and Juniata Valleys Embraced in the Counties of Mifflin, Juniata, Perry, Union and Snyder in the Commonwealth of Pennsylvania.  Everts, Peck & Richards, Philadelphia. 

7.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com), including these items in a file submitted by Ray KOHLER:

BIOGRAPHICAL INFO [from the Biographical Encyclopedia of the Juniata Valley (1897, J.M. Runk & Co.)]: SAMUEL DIMM, born in Juniata County, who, after receiving a good English and German education, taught school for several terms in Greenwood Township, Juniata County. He spent some time in distilling and milling, and was also one of the most extensive farmers in Greenwood Township, devoting much attention to raising fruit and rearing improved stock. He married Leah, daughter of Christian and Barbara (Moyer) Shellenberger. She was born in Juniata County, where her father was a farmer. Mr. and Mrs. Dimm had seven children: Joanna, deceased; Jennie (Mrs. Ferd. Rinehart), of Illinois; Wilson W., postmaster at Van Wert; Franklin, a resident of Kansas; Irvin, of Lewistown, Pa.; Benton T., of Millersburg, Dauphin County, Pa.; and Simon C., bookkeeper for the Omaha Iron Company, Omaha, Neb. SAMUEL DIMM took an active part in all public affairs. He held numerous official positions, among which were those of school director, supervisor, and constable. He was a Democrat. He was also prominent in the affairs of the Lutheran church, in which denomination he held the offices of elder and deacon. He died in Kansas in 1887. His aged widow still survives, residing with her married daughter in Illinois.Page: 888-88

OBITUARY for LEAH SHELLENBERGER DIMM LEAH SHELLENBERGER was born at Evandale, Pa., April 29, 1822; died at Harris, Ill., Feb 24, 1904, aged 81 years, 9 months, 23 days. She was married in 1844 to Samuel Dimm. They resided in Juniata County, Pa., until 1880, when they came to Farmer City, Ill., later removing to Kirwin, Kansas, where the husband died. After his death Mrs. Dimm live with her daughter, Mrs. Fred Rinehart, until her heavenly Father called her to her eternal home. She joined the German Baptist Church in young womanhood and ever remained a consistent Christian, being a member of the church at Mansfield at the time of her death. The infirmities of age gradually laid hold upon her until she calmly entered into peaceful rest. She was remarkably patient and bore her afflictions with cheerfulness and resignation. In character she was unassuming, uprightness being self-evident in all her conversation and deeds. She is survived by six children, nine grandchildren, two brothers and one sister. A daughter died in childhood. The living children are: Martha Jane, wife of Fred Rineheart, of Harris; Wilson W., of Van Wert, Pa.; Franklin P., of Oleta, Oklahoma; Irvin C., of Lewistown, Pa., and Simon C., of Denver, Colorado. Her body rests beside her relatives in St. Michael's cemetery, near Millerstown, Perry Co., PA