Infant/Child pads would be used on individuals up to 1 year.  pediatric pads at 1 year to 8 years.  Adult pads are anyone over 8 years of age.

now when we talk about age we also talk about size  common snese would be the deciding factor.  If the 7 year old looks like they are 9 then use adult pads.  If a 8 year old petite girl looks 6-7 then use pedi pads.  

If you only have adult pads they can be used on any age group.  The pads are programmed to shock a certain energy setting..  The adult pads would stop anyones heart in order to restart it.  But the infant/pedi pads deliver a smaller energy dose and they would not convert the rhythm into an organized rhythm.