I’m kinda new to blahtherapy and the first time I tried to be a listener someone asked me for my gender and age? Is this the usual thing? I feel like it’s unecessary and I was kinda pissed off but because they needed to vent I didn’t say anything and just left.
I don't think it's the usual thing, but some venters do it, especially if they're looking for a specific perspective on something. Like, a person having problems with their girl/boyfriend often look for a girl or a guy to get another gender's perspective, or people who are having problems at work might only want to find someone.
If you don't want to give out that information, you can say something like "Sorry, I'd rather not say". Just be aware, though, that the venter might disconnect, because they're only looking for someone in a specific category.
Don't get discouraged, though. If you're afraid someone might not take you seriously after hearing your age, say something like "15, but I've got a lot of experience." Honestly, it's more about the connection between you and the venter than anything else.