The written word ぴえん seems popular lately, but does it sound childish? I feel like only people under 20 might use this, or someone trying to appear cute. Is this impression accurate?
Real small children don't use ぴえん, so "childish" may not be the right adjective to describe this. It's a fairly recent slangy buzzword used mainly by (female) teenagers and some otaku. As of late 2020, it's already becoming outdated. 男女問わず幅広い世代に浸透した is definitely an overstatement. Someone who are over 25 and use words like these intentionally would probably look like a try-hard (it depends on their personality, of course).