I've had a look at some more censuses and I think your first assumption was right - that Edward was bapt 17 Oct. 1847 Maughold, son of Edward Christian and Jane Corkill. Both Edward and Agnes were pretty consistent with their ages given in the censuses. If he wasn't known in Lonan perhaps he just didn't bother getting permission to marry underage in 1866.
Agnes Skillicorn bapt Lonan 3 Sept.1847 daur of John Skillicorn & Margaret Kermode.

In the 1851 census Edward was with his family at Ballabroie, Lezayre, aged 3, and all family members b. Maughold except Hugh aged 4 months, b. Lezayre.
Maughold bapts:
John Edward son of Edward Christian & Jane Corkill bapt Parish Church 19 May 1840 [John aged 10 in the 1851 census]
Cath Jane daur of Edward Christian & Jane Corkill bapt Parish Church 16 Mar 1845 [Kitty J aged 5 in 1851], and Edward. I couldn't find a Maughold bapt for son William aged 7 in 1851.

Maughold marriage 4 Apr 1863:
John Edward CHRISTIAN Full age Bachelor, Labourer, Maughold [father] Edward Christian, Labourer, and
Priscilla WATTLEWORTH Full age Spinster, Maughold [father] Charles Wattleworth, Joiner.

In the 1871 census Edward & Agnes are at 1 Looneys Court, Ramsey, aged 23 & 22, with Edward a laborer b. Maughold and Jane & children b.Lonan. Son Edward 4 [Edward Charles bapt 17 Feb.1867 Lonan], son Evan 2 [Evan bapt Ramsey 7 Mar.1869].
You saw them in 1881 aged 32 & 33 with Edward now lead miner, son Edward Charles 14, Tailor's Apprentice, and 5 other children.
In 1891 they are in Lonan at 3 Croit-E-Callow, aged 42 & 43, with several more children all b.Lonan. Several of their children are on Brian Lawson's birth regs website, all regt Lonan: Agnes Marga. 15 Oct 1889; Allan Wilfr. 14 Aug 1879; James Henry 9 Oct 1883; John Isaac 25 Mar 1886; Oscar Freder. 19 Jul 1881.

In 1901 they are at The Shore, Lonan, with Edward & Agnes both aged 53, Edw. lead miner b. Maughold, son James H. 17 lead miner, Agnes M. 11, and Clara J Lewthwaite aged 5 Gr. Daur b.Lonan.
Brian's births has Clara Jane LEWTHWAITE, b. 29 Aug 1895, regt. Lon., father LEWTHWAITE, Alexander, mother CHRISTIAN, Sarah Jane, and with the same parents also Elise Chri. LEWTHWAITE, b.4 Oct 1897 regt. Dou.
Marriage: LEWTHWAITE, Alexander, 4 May 1895, D, CHRISTIAN, Sarah.
Sarah Jane Christian bapt 1 June 1873 Laxey [aged 7 in the 1881 census].

Sue