Seriousness "traits":
1. Having nothing insignificant (such as bravado) to prove to anyone. 12 year olds have got something to prove. You sir have not.
2. Talking less and listening more. I remember when I was a kid I'd have the urge to say things and so I'd blurt them out. Once you've got a little experience though, you can perform an analysis of the situation and think for a brief second whether that dumb question in your head really matters whether it's answered or not. That's not to say that men of wisdom don't fuck about or tell jokes. They do. A lot. They're just a bit less hit-and-miss because they've thought about whether they're worth sharing or not.
3. Talking about serious things. It helps if you're into politics, science, economics, etcetera. You don't have to berate people about the "strong opinions" you hold or talk "seriously" about said serious issues. Just make sure that the jokes you're telling are based on the stupidity of politicians or recent events, rather than on "lol so random amirite guise?".
4. In addition to talking about serious things, try doing a few serious things as well. Political engagement is the best way to do it, but if you can't stomach that kind of crap then pick some other issue that affects people's lives. Try doing something about the poverty in your city (or some other shit you care about) for example. It doesn't matter how significant your contribution is, say if you volunteer to help a charity store with moving stock around once a week. Nobody is going to say "anon is such a loveable idiot, what with him helping worse off people than himself".
TL;DR
Pick your battles.
Pick your words.
Pick your subjects of discussion.
Don't give me that "I'm too forthright or impulsive" crap. *Just a little*, just a split second longer; Think A Little More.