Do they grow into it? Then they can be nabbed early, before they can defend themselves not only because they don't know any better but because they are not strong enough.
I decided that no one gets any powers before the age of seven, and it's rare until fourteen, and none too common before twenty. BUT -- if they become immune, that would see lots of people ensuring that children are exposed young. The government doesn't want too many ultrapowerful people AND has the defense that it can go haywire badly; people don't want their children's lives disrupted by getting powers; etc.
Then, not everyone exposed is affected. So, it's like the Spanish influenza: mostly affects adults.
Except that I was thinking that the children of superpowered grow up into being superpowered. That'll be fun. Some people will try to get their children the constant level of exposure.