I’m a guy who looks somewhat young.
If I were a computer consultant, this would not be a problem, but, I’m in a profession where people want and expect you be somewhat seasoned.
So, if there are any dopers out there who are masters of fashion / disguise / makeup / whatever, what can I do to appear older?
Mustache. Beard. Tie.
Oh, and add a set of unattractve eyeglasses.
Round lenses with thick, round, black frames. Or “birth control” glasses will work, too.
I echo the moustache and/or beard, perhaps the glasses. They sometimes just make one look nerdy not necessarily older unless you get the right pair of glasses. A pipe can give a little age. You don’t need to smoke it. Just sort of carry it in your hand and mouth.
However, be prepared for people asking if you are ever going to smoke it or offer you a light. Saying you are trying to give up smoking is a good response.
TV
Facial hair in general can help, but the wrong choice can work against you. The modern concept of the goatee (which isn’t really a goatee, but I won’t go into that) will have a tendency to make you look like you’re trying to look trendy, which will (to many people) make you look younger. IMHO, a full beard, complete with moustache, is the best way to go.
Scars tend to age the face as well, but as I can’t really think of a good way to go about acquring them, that’s best left up to Providence. 
Cardigans and tweed
Point 1: You didn’t say how old you are & I can’t see you, so not much we can do in the way of image consultation…
Contact a men’s clothing store, a real store, not Kmart, ask the haberdasher for clothing to make you look more mature.
Find a hairstylist ask for a hair style to look more mature.
Join a gym, ask the trainer for a program to make you look more mature muscularily.
Get a tan.
Find an image consultant.
Remember Point 1.
Personally, I’d say nix the pipe. It will make you look older, but it will take about ten years, and cigarettes would probably work better for that. As far as just carrying it around goes, my sense is that pipes have become so rare that out of the hands of true elders, they are a bit of a joke. If I were consulting a youngish financial advisor with a pipe in his pocket, I would be unable to repress the image of that same advisor reclining in a smoker and cravat, blowing bubbles.
Stick with the hairstyle, beard, glasses, and clothing. All of these sound like great ideas.
Have you considered carrying, not a pipe, but a picture of a pipe?
[Ron Palillo voice]Ooh! Ooh! Ooh! I know![/Ron Palillo voice]
How about working on you diction, vocal tone, and stance. I suspect that could add years.
In hopes of receiving more detailed advice, I will say that I am 30 years old and average-looking (although I am frequently told that I have nice eyes). I’m below average height (5’8") and caucasian. I’m stocky in build - fairly muscular, with a little extra fat around the middle, but not a lot. I have a full head of hair, and no gray. My hair is so thick that I wear it very short (not quite a crewcut). Oh and I have fair skin and burn pretty easily, at least on my face.
People think I’m still in my 20s and are surprised to hear that I’ve been to college and grad-school and that I am married with two children.
My first instinct was to see this post as the opportunity for some smart Alec remark along the lines of: keep your mouth shut. I see however from the tone of the post and the follow-ups that this is a serious question that deserves a serious response. So, for whatever it may be worth, here is some input from a codger who has had some experience in interviewing and hiring young people.
You are probably a lot more worried about how young you look than anyone else. As long as your behavior and demeanor do not betray you as a callow and feckless youth, your youthful appearance will operate in your favor. Old guys like to hire young guys, if they are hard working and don’t have to be told twice what to do and how to do it.
Dress decently. Find out what the uniform of the day is where you are interviewing and imitate that costume. Look like you already fit in. If it’s a suit and tie place, wear a suit and tie. If it’s an open collar and khakis place, wear open collar and khakis, but neat and clean.
Be respectful. Yes, sir/ma’am, never did anyone any harm.
If asked how old you are, tell ‘em, without getting defensive or apologetic about it.
Forget facial hair. If you are all that young, any beard is just going to look silly and create the idea that you are trying to fool someone about your age. It is not nice to deceive a prospective employer, and even worse to be perceived to be trying to deceive. Whatever you do, don’t pretend to be something you are not.
If you have a diasthyma (space between front upper teeth), get it filled. If you are shorter than 5’9"get shoes that make you taller. If, in the genetic crap shoot, your were not blessed with “good looks”, a weak chin, etc., consider plastic surgery. Pardon my crassness, but we live in a superficial society obsessed with looks. It’s not for me, personally, but whatever gets the job done.
If your beard isn’t very thick, you are gonna look ratty for several weeks. Apply minoxidil to increase growth rate.
Posture is key, as is Gelding’s point about clothing. If you’re kinda chunky, only wear dark clothes.
The yes, Ma’am, no Sir bit might backfire though. If these people are 30 years older than you, definitely, but don’t appear obsequious.
Attempt to think of any comments or questions that appeal to a person’s knowledge. It strokes their egos. If the opportunity presents itself, compliment them on a well tailored jacket or jewelry for women. Know your stones so you can ask if it’s a this or that. People don’t compliment each other enough. I do it a fair amount and half the people are taken aback, and then they relax and ususally have some pleasant comment about how they got it or whatever. It sets them at their ease.
If it’s the suit and tie crowd, for an interview, I wear a double breasted Very dark blue blazer and grey slacks. No cutesy ties.
Look people in the eye when they are talking to you and always get their card.
I get that a lot, too, but it has more to do with my behavior than my appearance.
I feel you, kneadToKnow. I am 37, reddish hair, big space between my teeth and totally rambunctious. People think I am 25. But I have this vile habit of using words people don’t know–I was raised by sespiquidelian narcissists–and they quickly box me into what I am; a case of arrested development. C’est la vie. The job gets done very well. Nothing is serious or sacred except for child abuse and rape.
Now, why don’t cannibals eat missionaries?
Because you can’t keep a good man down.
Lucwarm, you didn’t say what trade you are in.
Hey! Shouldn’t you at least buy me a drink first?
Sorry. One of those West Caost colloquialisms sneaking in.
Sure. Perfect Manhatten with rye whiskey for me. What’s your sign?