Newer platforms will tend to skew younger for sure. I was too young to get into irc really but even when I was young instant messaging services like MSN Messenger were vastly more mainstream than forums. So maybe it's not really based on age.

But I do see, generally young, people join niche discords that don't necessarily see 24/7 activity say something like "Hey anyone here?" then a few minutes later "lol dead server". I think it's because for a lot of younger people they mostly want the socialization aspect over just a place to share about a hobby. They go on discord when they want to talk to people.