the census at large accross the world is generally based upon "age known at last birthday", however, if one has a christening certificate instead of a birth certificate and for example one was christened a year or two after one was born then do you know how old you are, and, so there is straight away an approximate age given...
However in the vast majority of cases in the 1841 uk census the age was anything up to 5 years out, and one notices in america it can be anything up to two years out
So if you are relying on the ages given in the census please dont, get the actual certificates or the christening details to assume a sense of correct age.
As for women or "minors" having children , well, unfortunately it happens, and in some cases where the parents are married off as young as 12/13/14/15 then one does stumble accross the odd child thereof where it seems to be incorrect.
best i can offer without finding all the records you have sighted or discounted
others may find or discount my opinion which is all good

marty