I am inside you, but never others,
Everybody always uses me every second, more than once,
When you want to ask a question, I am something good to bring up,
If you change me even a little, I become an adult, and if you do it again, I become a kid,
I am a spy for both groups, but one group is called more often, despite having less members,
If I am lucky enough to be on a flag, I will be red and yellow, but in real life, filled with color, I am blue.
What Am I?
(The following is NOT part of the riddle:) Please explain your reasoning for each clue and conceal our answer/reasoning behind a spoiler.