Dear Dami,
I had to translate your question via google translator. So I hope I was able to understand your need correctly
Lets say your question about the age is Q1.
The following questions for those beeing 19+ is Q19
The question for those beeing 29+ is Q29
Lets assume someone beeing 32 years old will answer. He is as older than 19 as well as 29.
So the relevance equation for Q19 has to be: (Q1 ge 19)
(You can extend this condition to (Q1 ge 19) AND (Q1 lt 29) if you do not want display this question for those beeing 29+)
The relevance equation for Q29 has to be: (Q1 ge 29) AND (Q19.value = '')
So if someone has already answered Q19 question Q29 will not appear anymore.
Hope this is what you need.