To Emeritus, Not a polite answer, but an answer all the same, thanks. So you mean that the 1901 Census, is for years ending and not beginning. Thank You.
The census recorded all the people who were alive and in the UK on 31 March 1901.
Somebody not born until 1913 did not appear in a census until the one held on 19 June 1921, provided they were still alive, and in the UK, on that date of course.