Spend some serious time thinking about whether what you do is reflective of someone who is older than 30 or 18.
You want people to see you as a business professional.
First, jmort253 briefly talks about this, but it really is important to dress like a professional. This is important. Really important. You don't say what industry you are in but you will almost always be treated more seriously by wearing nicer clothing. A polo/khakis in an environment with jeans/tshirts. Perhaps even a suitcoat. A trivially easy way to demonstrate this is to go to a car dealership dressed in jeans vs business dress. Without fail, no one has ever taken me seriously while wearing jeans (just another kid looking at cars...), but 100% of the time, when wearing business casual, I get immediate attention (oh, this guy is 1. serious and 2. probably has money!).
Second, look through this list and make sure you are really aware of your personal tendencies. If you act young, and look young, people will treat you like you are young. If you act older, even if you look young, you will get taken a lot more seriously.
- Do you say "like" or other filler words all the time? If so, stop this as soon as possible
- Do you talk fast? Spend time concentrating on slowing down your speech (this tends to help with the above too)
- Do you play on your phone/check it often? Stop doing this if so.
- Do you mumble? Or speak clearly? Concentrate on speaking clearly.
- Do you look people in the eye and practice active listening? Or do you fidget when people are talking to you?
- Do you take notes when people are talking to you about technical subjects? Or just assume you will remember things?
- Do you ask questions of things you can easily solve yourself? Don't be a help vampire.
- Do you show up to work really tired? Or talk about... weekend activities?
- Is your hair an unkept mess or combed?
- If you are male, you may find well-kept facial hair will make you look older - only do this if it looks good though and the environment is appropriate (some industries care much more about this than others). If you are unsure if it looks good, ask someone
- Figure out what length hair makes you look older. If I have my hair short, I look YOUNG, but putting it a longer length makes me look older
- Do you constantly deliver on projects? Coworkers are likely to take you much more seriously when they know you not only knowledge but ability to apply and execute.
- Do you slouch or have good posture?
- Do you get upset about little things or have troubles with authority?
- Are you confident? Or timid/shy? People take a confident person a lot more seriously.
- Be careful using emoticons in email/IM
- Do you use IM (if your office has it) in a way like your older coworkers?
Don't give people reasons to think you are a teenager.
The point of all this is you want to be perceived as a professional. You cannot necessarily change your physical appearance. But you can change the whole host of mannerisms, quirks, and personality traits which will either reinforce the age your physical appearance suggests, or, counter it.
As humans, we make a lot of assumptions based on how people act and talk (in addition to simply appearance). Many of your coworkers might have teenager children. If you act similarly and look similarly to those children you should not be surprised if they treat you in that way.
Yes, people judge at first glance based on age. But how we act and present ourselves has a much larger effect than simply what our physical appearance represents.