1911 Census of England and Wales, General Report with Appendices, Table 123: " Proportion of Lunatic, Imbecile and Feeble-minded per Million of the Population".

All
Ages.
AGES.
Under
5
years.
5— 10— 15— 20— 25— 35— 45— 55— 65— 75— 85 and
up-
wards.
Lunatic 2,957 6 34 76 380 1,175 2,672 4,807 7,081 8,788 9,087 8,754 8,087
Imbecile 704 67 371 579 799 911 858 876 853 914 854 1,050 1,358
Feeble-
minded.
830 74 452 876 1,222 1,025 853 740 788 1,004 1,519 3,006 6,073
All forms of
Insanity.
4,491 147 857 1,531 2,401 3,111 4,383 6,423 8,722 10,706 11,460 12,810 15,518