"Family historians need to understand that the ages given in 1841 Census may be 4 years 'out', because when a person was over the age of 15 years, the age was rounded down to the nearest 5."
Only should have been rounded down, not necessarily was.
I have come across 1841 returns both with the ages rounded up and with them as accurate, as well as rounded down.
Oh, and with the later censuses, either some people were really ignorant, or being economical with the truth as I have found age discrepancies for people between 1851, 1861, 1871, 1891 and 1901. Usually ladies and usually making themselves younger, but with at least one person, somewhat random.