How to gain credibility as a young professional?

Confidence is key (and I know it's an easy thing to say and hard to actually do). As you know, in our position we're often one of the youngest people in the room, if not the youngest - you need to be sure not to remind people of that. There's something to be said about the tone in the way things are said, such as don't trail off with your answers / when you speak.

For me personally I find it helpful to reflect after every meeting, presentation, etc. and think about how I carried myself. There's often a natural tendency to defer to higher ups in the room or hedge and say things like "I think x, but will let [senior person] confirm" or "[senior person] will know this better, but I believe x" because you want to protect yourself in case what you say is wrong, and are using your lack of experience as a way to soften the blow. Issue is by doing that, you're reinforcing other people's notions that you aren't experienced.

Otherwise there are certain things you can do when you introduce yourself or are talking about your past experiences to frame things in a certain way that don't call attention to your age. For instance, I don't say "I spent 3 years in investment banking before joining [current firm] just under 2 years ago" as that immediately is a reference point for how old I am and how many years of experience I have. Instead, I say something more along the lines of "I was previously an investment banking associate at bank before joining [current firm] a few years ago." Why? Because it's true that I left banking as an associate, but that could mean a lot of things to the other side - could be I was a recent promote, post-MBA associate, or whatever, and by also not explicitly calling out when I joined, it's harder to immediately peg me as a "junior person."

I know your question was more about external perceptions, but internal perceptions matter too. Do things like ask for more responsibility on projects or see if you can have a greater role on IC discussions - by doing so you can practice having more of a voice in a "less risky" environment, and more importantly you build your team's trust in you and they will start treating you like more of peer when you are in external meetings.

Now this is really more of a sidebar but it relates to the internal perception point I mentioned above. This is advice that was given to me (which I know I certainly need to work on still - big area of improvement), but at the junior levels it's really easy to bitch and moan and complain about things in a "public" way with your peers. By "public" I'm not talking about complaining out in the open where people can hear what you are saying, but if something happens in a meeting and you are frustrated or you get an email blowing up your weekend, don't immediately grab some fellow associates and hop in a conference room to vent. People may notice actions like this more than you think and you don't want to be known for being part of the junior cohort that is always gossiping and banding together because they are frustrated, and as you progress upwards, it's important for people to view you as an internal leader. Now this doesn't mean don't complain at all as everyone needs an outlet, but do it truly in private or find a way where you are only confiding in a handful of people and not bitching to your entire class. And I think I saw this mentioned by somebody else on this forum before, but try not to complain downwards.