Birthdates from tombstones are like birthdates from death certificates:  they depend on the knowledge of the informant.  As most women are lying about their age by mid life, the person ordering her tombstone may not have known her true age.  She was probably fibbing about her age in the 1850 census as well, so we can only roughly know what her age was, but the earlier date is probably closer to the truth.

1.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Standard, Gideon

Wiatt, Sinah

9 Dec 1823

Todd Co.

2.  1830 Census Index (online at GenealogyLibrary.com):

1830 Fed Census	Standard, Gideon	KY	Todd Co.	p. 375
1835 State Census	Standard, Gideon	IL	Morgan Co.	p. 044

3.  1840 Census Index (online at GenealogyLibrary.com):

Standard, Gideon

IL

McDonough Co.

p. 204

McDonough Co. was formed in 1826, so this was a genuine move from Morgan Co. to McDonough Co., not just a change in county name.

4.  1850 Census Index/Microfilm (online at GenealogyLibrary.com):  McDonough Co., IL, p. 311, 1135/1135, enumerated 15 Oct 1850 (extracted by Diana Gale Matthiesen):

Gideon Standard	50	M	Farmer	4000	Ky
Sine       "	45	F			Ky
Mary E.    "	23	F			Ky
Julia A.   "	21	F			Ky
Sherrod R. "	18	M	F.		Ill
M.J.       "	16	M	F.		Ill
Midham C.  "	14	M			Ill
James C?   "	12	M			Ill
George W   "	10	M			Ill
Nancy E    "	8	F			Ill
Martha E?  "	6	F			Ill
America    "	3	F			Ill

Living next door to Gideon's son, John B. STANDARD.

5.  Anon.  1885.  History of McDonough County, Illinois... Continental Hist. Co., Springfield, IL (online at the McDonough County USGenWeb site):

J.B. Standard came to McDonough county, in 1836, and settled then, on section 24, Industry township, where he resided until 1851.  He then moved to section 19, which has since been his residence.  He was born in Kentucky, June 23, 1824, and is a son of Gideon and Sina (Wyatt) Standard.  He came to Illinois when a boy of eight years, and lived in Morgan county until he came here.  Mr. Standard is one of the most extensive land owners, and one of the wealthiest farmers of McDonough county.  He has 1,050 acres of land, all in a high state of cultivation, except 188 acres of timber.  His improvements are among the finest in the county, his residence, barn, and other buildings being commodious and substantial.  He keeps a large number of cattle, horses, and other stock, and carries on farming on an extensive scale.  He began life poor, and his success is an example of what may be accomplished by industry, perseverance, and good management.  Mr. Standard was married in 1849, to Jane Allison, daughter of John Allison, of Pennsylvania.  By this union there are seven children — Mary, wife of Jerome Jones, of Scotland township; Margaret, wife of Thomas Jones, of Clay county, Nebraska; Julia, wife of E.F. Wheeler, of Kansas City; Malinda, wife of George W. Russell, of Bethel township; Gideon, living in Industry township; John, also in Industry; and George, living at home.  Mrs. Standard died September 25, 1876, and was buried in the Vance cemetery, near Industry.  In September, 1880, Mr. Standard was married to Mrs. Martha Duncan, widow of Dr. Duncan, and daughter of Morris Merrick.  By his second marriage, Mr. Standard has one child — Fannie.

6.  LDS.  Ancestral File (online).

7.  Family Group Sheet by John Charles Tippet.

8.  Family Group Sheet of Gideon T. & Sinia J. (WYATT) STANDARD. Everton's Family File Vol. 4 (Broderbund CD-14; Disk 1).  Says 6th child is William Columbus STANDARD.

9.  LDS.  IGI (online).  Kin?

Marriage	Sina WHITE	Oliver SHREVE	1796	Of Todd, Kentucky
Relative	Sina WHITE	Heber J. GRANT	1796	Prob. Todd, Kentucky
Marriage	Sina WHITE	Oliver SHREVE	1815	Todd, Pulaski, Kentucky
Marriage	Mrs Sinah WYATT	Joshua WYATT	ABT 1769	Northampton Co, Virginia
Birth	Sina WHITE		1790	Loudoun, Virginia
Birth	Mrs Sinah WYATT		ABT 1750	Northampton Co, Virginia