I had this issue when I first started work. One client asked how long I had been doing the job, I inferred he thought he had got someone in their first day of work.
By a funny life coincidence, I found myself in the same exact room, conference and situation 2 years later, just a different client. They did did not ask how long I had been in the job and I realised I had changed.
Some of my changes were physical, some mental. Originally, I wore suit skirts and suit dresses (suit environment) with coloured or fashionable tops. I worse makeup and jewelery, heels - I liked nice. After 2 years I was more tired by the job and life, so some of the makeup had gone, and my clothes were more neutral, than fashionable. Still nice to me, but less girly. I had long hair, but didn’t style it every day anymore to something feminine (even a ponytail) - back in a bun or chignon.
I also had/have a light voice - I made sure to speak less, but with authority, never raise my tone at the end of a sentence (sounds like you
are unsure/a question), and to be less friendly/helpful around work colleagues. You still want to be a good worker, but you need to shed the juvenile vibe. As someone said - think less bouncy puppy, keen to help.
I’d recommend just WATCHING office dynamics too. Watch and try and find and see how they treat another young person, and then an older person, or someone you’d like to be treated like. Hopefully you’ll see the mannerisms that make someone seem young, inexperienced and like they can be walked over abit.
20 years on, I now do everything I stopped doing - I wear heels again, red lipstick and my long hair out or in ponytails whenever I want. My experience comes out in my tone and probably
my wrinkles and greying hair! But I can dress however I want. But I look and see the same for young girls (or even not so young girls) coming through and see it’s still the same