Various jurisdictions including Canada, England and Wales and Washington have laws which state, apparently categorically, that children under a certain age cannot commit crimes or be convicted of crimes. E.g. Canada's criminal code section 13 says:
No person shall be convicted of an offence in respect of an act or omission on his part while that person was under the age of twelve years.
Does this mean that a minor under the specified age could "openly have, say, a contract killer career, retire at [the specified age] and walk free"?
What legal mechanism would stop a child that is younger than their jurisdiction's minimum age for criminal liability from doing this?
There may very well be legal consequences for the minor's parents or legal guardians, but this question is specifically about the legal consequences for the minor.
(This question is a follow-up to a discussion on this other Q&A, where @Greendrake proposed this hypothetical example.)