What about the ages of Joshua and Manerva? We know Joshua's birth and death dates; they are recorded on his grave marker.
We know Manerva's date of death but not her birth date. We do have census records that give us a good idea within a couple of years. 
Why bother establishing that Manerva was born in the early 1820's rather than the 1811 date that many Web sites are using? I'll answer that question below. But first, look at these 3 Census records.
Joshua was born on December 24, 1820; the Census was taken in warm months before the cold weather.  

1850 Census Smith Co. TN Dist. 11
Joshua and Manirva Dickens
Joshua 26 M (should be 29)
Manirva 26 F
Martha A 5 F
John W 4 M
Sam'l R. 4 M
Clarissa 2 F
Wm V. 7/12 M (pg0221b)

My notes:
Joshua’s birth date from his headstone,
 was Dec. 24, 1820.
 Dec. 24, 1850
-Dec. 24, 1820
On Census day (before his birthday)
Joshua was 29 not 26.
Manerva was born in 1821 or 1822.

1870 Census Smith Co. TN Dist. 11
Dickens Joshua 50 M Farmer
Dickens Manervy 49 F House Keeper
Dickens Martha 24 F Ast House Keeper
Dickens John 23 M Farmhand
Dickens Eliza 18 F Ast House Keeper
Dickens Shep 15 M at Home
Dickens Amda* 10 F at Home
Dickens Fanny 8 F at Home

My notes:
 Dec. 24, 1870
-Dec 24, 1820
Joshua was 49 on Census day.
Manerva was born in 1821 or 1822.

1880 Census Smith Co. Tn. Dist. 11.
Joshua DICKINS Male 59 TN Farmer
Minerva DICKINS Female 58 TN Keeping House
John DICKINS Son 33 TN Farm Laborer
Lisa DICKINS Dau 22 TN At Home

My notes:
  Dec 24, 1880
-Dec 24, 1820 
Joshua was 59 on Census day.
Manerva was born in 1821 or 1822.

Notice that in the three records, Manerva is listed as
the same age or one year younger than Joshua. So:
In 1850 her birth year would be: 1821 or 1822.
In 1870 her birth year would be: 1821 or 1822.
In 1880 her birth year would be: 1821 or 1822.
Manerva died April 3, 1881.
I've located some excellent Web sites that diagram the entire Young family lineage. (I have a daughter-in-law
who belongs in the Young family.)
Unfortunately, sites list Manerva as born in 1811. Why not just ignore the exact birth year? We know who she was. 
What does it matter whether it's 1811 or ten years later?
I'll explain below.

(The following is based on an Internet site.)

2. EMILY YOUNG, b. 1814.
3. WILLIAM FREDERICK YOUNG, b. 15 Jun. 1816, d. 05 Aug. 1889
5. ISAAC GEORGE YOUNG, b. 27. Jan. 1818, d. 19 Sept. 1884;
7. JAMES PRESTON YOUNG, b. about 1827.
8. VANSEN (or VANDER) YOUNG, b. 1837, Putnam County, TN.

(The following is based on an Internet site.)
SAMUEL YOUNG (Son of William Young and Elizabeth Huff) was born
6 Jul. 1787 in Virginia and died in 1857 in Jackson County, TN.
He married (1) 24 Aug. 1813 in Sumner County, TN, ELIZABETH GEORGE (daughter of Isaac George and Elizabeth Gregory). Elizabeth died in Tennessee. He married (2) MARTHA "PATSY" FITZGERALD. He married (3) ELIZABETH DICKENS.

Young, Samuel - George, Betsey          24 August 1813

The marriage of Manerva's parents is recorded in Sumner Co, TN (Betsy is a nickname for Elizabeth).
Now look at the published birth dates of their children: 1811,  1814,  1816,  1817,  1818,  1825,  1827, 1837
1811 was about two years before they married and 1818 - 1825 is a seven year gap where Manerva should be.

Earlier, I said that I would explain why Manerva's birth year should be corrected. If she was born in 1811, which was before her parents married in 1813, she was an illegitimate child and there would be a Court Record of her illegitimate birth. It was called the "Bastardy Law" and it was strictly enforced. A 'bastard child' carried a stigma throughout life. Until just a few years ago, that stigma persisted (I was born in 1932 - I remember it very well) even if there were no penalties required.

Several years ago, I knew a fine man who, it had been rumored, was born of unwed parents. There were always whispers behind his back that he was a 'bastard' child. Shortly before his death, the State released his adoption papers. It was then found that he was a "depression baby". His parents had given him and his several siblings, for adoption in the Great Depression of the 1930's because they couldn't afford to feed them. Earlier, the  law had prevented the State from releasing the information containing his true identity. People thought he was an illegitimate child and the stigma of that erroneous idea followed him all his life.

Now, laws have changed. Today, there are many children born outside of marriage and there is no such stigma attached to the child or the parents. Not so in bygone days. Type Bastardy Law in your search engine and see the many references to it. Meanwhile, read the excerpt below: 

"Bastardy Bonds
This bond was required from father of illegitimate children. This bond was usually made with County Court where the mother resided. The intent was to protect the county from being forced to support the child. Occasionally, these records still survive.

John Haywood’s manual for Tennessee Justices of the Peace (1810, reissued 1816) lists bastardy as an un-indictable offence which can be tried before any two justices. If found guilty the father is to be brought before the full court to provide bond and security. This would be the County Court comprised of all the justices, not the Superior Court. The bond is technically a "bastardy" bond, not a "bastard" bond. (Charles A. Sherrill, Tenn. State Library & Archives.)"

Bouvier's Law Dictionary
1856 Edition

Unless someone can produce a Court record of Mr.Young paying a Bastardy Bond, Joshua's wife Manerva
was not born out of wedlock in 1811. She was born in 1821 or 1822.
Here are a couple of other good references:
Joshua Sr. Page 2.
Joshua Sr. Page 2.
                           Dickens of Hurricane Creek
        Maggart, Sullivans Bend, Elmwood, Chestnut Mound, Smith County, TN
Spelling for Manerva is that which appears in the census.