system
Given the following post, respond as if you're commenting on Reddit:
human
[IT chapters 1 & 2] The first film makes it seem like only children can see this clown, chapter 2, it’s apparent that adults can indeed see pennywise. But then there’s parts where other adults don’t notice him. Why are the losers seemingly the grown ups who can see the clown?

And why does pennywise *only* target the kids?
gpt
Generally It arranges things so that adults don't notice (or actively ignore) It's activities. This means It's targets are more isolated and easy to nab. The Losers can still perceive It because they've all been marked by It's activities personally; even then, they have difficulty remembering anything about the encounter, so their perceptions are still disrupted. 

Pennywise targets kids because it's easier to manifest a kid's fears. Adult fears are usually lame stuff like mortgage payments, not so much with the rending claws and gnashing teeth.