GMAT: The basic idea of the GMAT is that it can't help you but it can hurt you, so you want it to be a non-issue. My target would be to be above the mean (which is somewhere just above 700). I don't think getting a score higher than that really helps, but if you're below say 650, the other parts of your application have to be much stronger.
Interviewing: You should prepare for your b-school interview like you would prepare for a job interview. It's probably going to be a lot less formal, but you have to come across very polished and well spoken. If you have any holes in your application (no community service, gap in your work experience, little leadership experience), expect to be asked. Definitely have a good, canned answer for why you want to go to business school and why you want to go to UCLA Anderson. Those are the two most important questions. I didn't do this when I applied, but I would practice answering these questions aloud and keep the answers to 1 to 2 minutes. Too short is bad and too long is worse. Interviews are the key component of getting a post b-school job, so UCLA Anderson wants to accept people who will be very poised and confident when they go to interview with Goldman/McKinsey or pitch their business idea to a VC.
Age: This is going to be more of personal choice and it really depends on your current situation. I would say that most people here are in the 27 year old range. I think if you enter UCLA Anderson at 30 you feel a little old and if you come in at 25 you feel a little young. Ideally, I would say that you should try to come into b-school at around 27. That way, you'll be in kind of the same "maturity zone" as the rest of your peers. More work experience definitely makes the b-school experience a better learning environment. You'll have your own experience to relate to when you're learning about different aspects of business, and you'll have better comments to share with the class during discussion. But there's a trade-off -- you also want to get into b-school early, so you can graduate and start working at a great place with a lot more responsibility/impact.
General advice: I think the best thing you can do is be really introspective about what you want to do with your life. Your "story" about why you feel business school is right for you based on your background, values, and goals is critical. Don't say what you think the admissions committee wants to hear -- I think it's obvious to them (because they've seen that 1000 times). And I think one mistake people make is that they feel that they have to be really unique or different in their application. Just do what you think is true to yourself -- even if it may seem boring or basic.
I hope that helps!
Disclaimer: I don't interact with admissions office very much, so these thoughts are based on my impression from getting in and from the other people at school, which I think are probably pretty accurate.