Hi! Congrats on landing the internship!
It sounds like you are off to the right start by simply being aware of how your clients might perceive you. I went through a similar experience myself in one of my first jobs. I was 24 and providing career counseling to men twice my age.
One important thing to remember is that the people who hired you believe in you. They know you are capable of handling the job. Try to be confident, and pay attention to things like your body language that might make you seem unsure of yourself. Most people start making judgements about other people before they even talk to them, so things like your posture & eye contact will be important.
Also, once you start working with clients, and they learn more about your personal story, they will see that you know what you are talking about and that you have the ability to be helpful to them. The fact that you've "walked in their shoes" will carry you far. It will help them to realize that you understand them.
Some other things that might be helpful as far as displaying a professional image would include little things like being on time, dressing professionally, keeping hair & makeup fairly conservative, and speaking professionally, even if you aren't given the same respect at first. I know it doesn't seem fair, but as young women, we are often judged more harshly at first over things like dress & how we present ourselves.
Also--consider your supervisors. If you are worried that they might perceive you as less than professional because of your age, then don't give them the chance to prove their assumptions to be correct. By doing things like turning in reports on time, speaking up at meetings, offering to help with extra projects, etc., they will see that you are willing to be part of their team & will start to treat you as such. Your clients will see this, too, and that will help them to see you are worthy of their respect, as well.
If you have a mentor at your internship, be sure to voice your concerns to them and invite them to give you constructive feedback about your professionalism & interactions with clients. If there isn't a formal evaluation process in place for your internship, then be sure to ask for an informal evaluation approximately half-way through the internship. One of the best ways to learn & grow is to get feedback.
Finally, if you do hear from a client (either directly or thru the grapevine) that they don't think you are capable of helping them, then I've found in my line of counseling that it typically is most helpful to politely confront them about their concerns. That way, if they are gently put "on-the-spot", they have to voice their actual concerns, and that can lead to all sorts of new pathways & conversations to help the counseling process along. It will also demonstrate your assertiveness.
Anwyays, my message is getting kind of long. I sure appreciate your question. Thanks for writing in. Best of luck in your new position, and please give us an update after you get going.
Readers--any more advice?
Thanks again!
Kristy