Pattern of asthma prevalence in children age groups

ISAAC studies have commonly used age groups of 6-7 years and 13-14 years for describing the burden, why were other age groups not taken into account ? Is there any scientific justification for that? Some studies have also used age groups of 3-7 years, 8-14 years and 15-17 years to find out the difference in prevalence using ISAAC questionnaire as study tool. Are they justified in that by choosing these age groups and if yes, what could be the possible explanation of that in we look at course of disease whether there is increasing trend as the age increases (3-7 years) and there is the dip in between years (8-14 years) and then again rise in the trend (15-17 years)?