As a seaman George (b.1857 Braddan) was probably away from home for the census.

Yes, a minor for marriage was under the age of 21.
The IGI has an extracted entry for MARY ANNE WOODWORTH Christening: 30 NOV 1862 Kirk Rushen, Isle Of Man Father: MATTHEW WOODWORTH, Mother: ELIZABETH CANNELL.

There is an 1881 census entry on FamilySearch.org with George's name probably misspelt:
Joseph MADDRELL Other M Male 30 Douglas, Isle of Man, England Master
George RIDCLIFF Other U Male 24 Douglas, Isle of Man, England Mate
John BENDING Other U Male 20 Exeter, Devon, England A B Seaman
John MALONE Other U Male 20 Drogheda, Ireland O R Seaman

Source Information:
Vessel "Maria"
Census Place Longbenton, Northumberland, England
Family History Library Film 1342229
Public Records Office Reference RG11
Piece / Folio 5087 / 112

Sue