A question about which language was spoken was asked in the 1901, 1911, 1921 and 1931 censuses (there was no census in 1941) - replies could be English or Manx only or Both. Few were Manx only (in those fiches I examined for 1901 I did not see any such) thus 'Both' is taken as indicating knowledge of Manx
Decline in enumerated Manx Speakers
Jackson makes a few comments on this decline - comments on the 1911 census had been made in Mannin in 1913; in terms of % of population 1901: 8.1%, 1911 : 4.8% 1921: 1.5%, 1931: 1%. By 1951 it had dropped to 0.5% and no language question was asked in 1981 (the last native speaker Ned Maddrell died in 1974); however Nationalistic sentiment, helped by Manx Government help in funding classes, has raised knowledge of Manx to around 2% of the population in 2001.
The decline from 1901 to 1911 is close on 50% - however when the age distribution of claimed Manx speakers is examined there is ground for suspicion that the figures for 1901 and 1911 are possibly not consistent and that those for 1901 are probably over-reported.
Age distribution of Manx Speakers in 1901 census - blue Northern parishes Michael, Ballaugh and part of Lezayre; Red - Rushen
The sample, made by noting replies on three 3 fiches, covers about 1 in 8 of the claimed Manx speakers - Rushen (which included Cregneash) was noted as possibly a stronghold of Manx, though here the age distribution peaked in the 60-70 age group whereas that for the Northern parishes the peak was in the 50-59 group. Assuming that the aggregate distribution was typical of the Island as a whole then life expectancy statistics would have indicated a greater number of Manx speakers in 1911.
There is a suspicion therefore that knowledge of Manx might have been over reported in 1901.
However the absence of young speakers of Manx is also obvious - though one 'joke' reply (that of a 1 year old who spoke both) was noted. One particular response - that of Henry Percy Kelly, President of Manx Society in 1918 is particularly interesting - born in Rushen in mid 1870's, both parents claimed Manx but of their children only he claimed Manx (and in fact won the Manx prize at KWC) thus confirming Jenner's comment that few parents were passing on the language..
The 1911 census as reported in Mannin shows more speakers of Manx under 10 in Douglas than in the rest of the Island combined (11 out of 19) which possibly reflects the activity of the Manx Language classes.
Age distribution of enumerated Manx speakers 1911
B. Stowell & Diarmuid Ó Breasláin A Short History of the Manx Language Belfast: An Clochán 1996 (ISBN 1-900286-02-5)
R.L. Thomson "The Manx Language" in A New History of the Isle of Man Vol 5 - The Modern Period 1830-1999 Liverpool: University Press 2000 (ISBN 0-85323-726-3) pp312/316
P. Gawne "Preserving the Future of Manx Gaelic" in Mannin Revisted Edinburgh: Scottish Society for Northern Studies 2002 (ISBN 0-9535226-2-8) pp173/183