Small world!
Here they are in 1851. Joseph age is definitely given as 22. This would make him 12 when he married, so I think that the enumerator must have misheard or just made a mistake.
HO107/1714 Folios 13 & 14 Pages 19 & 20
Aldenham
Herts
ROMNEY(sic)
Joseph, Head, Married, 22, Ag Lab, b Hatfield
Hannah, Wife, 26, b Sandridge
Thomas, Son, 10, Scholar, b Wheathampstead
William, Son,9, Scholar, b Shenley
James, Son, 7, Scholar, b Aldenham
George, Son, 3, b Aldenham
Ann, Daugher, 1, b Aldenham
And in 41. The 1841 census does not give relationships within families, marital status or exact place of birth. It only indicates whether or not someone was born in the census county. Adult ages are often rounded down to nearest 5.
HO107/442/8 -9
Gustard Wood Common
Wheathampstead
HertsAll born in Herts.
EDMONDS Elizabeth, 50, Lace Maker
RUMLEY (sic) Joseph, 20, Ag Lab
RUMLEY (sic) Hannah, 20
RUMLEY (sic) Thomas, 6 months.
They were clearly both very young when they married and it would probably be well worthwhile ordering their marriage certificate if you do not already have it. It would be very interesting indeed to see how old they told the vicar they were!
Dec Quarter 1840
St. Albans Registration District. Volume 6 Page 779 http://www.gro.gov.uk/gro/content/certificates/Rick