I've calculated the age of the child by doing ( year_of_birth - date_obs)
What I want is : if the child has more than 5 years I'll ask him some questions. So I want to use the "isMoreFive" in relevant line.
From what I understand, you're using isMoreFive in a relevant column but that row isn't showing and hiding as you'd expect.
The relevant column should always have true/false (boolean) expressions in it. isMoreFive will either have the value 1 or the value 0 and therefore can never be false. You probably want isMoreFive to be just ${ageInYears} >= 5 or to use that expression directly in the relevant column without introducing a new calculate.