Can I investy in Vanguard when i get 18?

What kinds of funds should I choose? When is the time to pay taxes for the profit?

Yes, but you can invest before that if your parents fill out tthe necessary forms. See this Vanguard link about UGMA/UTMA accounts.
https://personal.vanguard.com/us/account...

If you are like most people you will invest part of your money aggressively in stock funds, and part conservatively in money market funds and bond funds. Vanguard has an on-line questionnaire which will give you an idea of how to do "Asset Allocation," determining how much to put in each type of fund.

I like index funds. Because of their broad diversification, you are less likely to have a dramatic drop in value. They also have the lowest expenses. For stock funds, I would suggest putting ~70-80% of your money in the Vanguard Total Stock Market Index Fund. and ~20-30% in a foreign stock index fund. I also like there target retirement funds. However, there are many different opinions out there on what the best mutual funds are. Read the links below and form your own opinion.

If you are working, and your company offers a 401K plan at work, try to invest the most you can. The money grows tax free, and some companies will match your contribution. Investing in a mutual fund IRA is also a good idea.

If you have high-interest debt, like credit cards, it is best to pay this off first before trying most of the investment ideas above. You should also have 3-6 months of salary saved up as an emergency fund in a bank or money market fund before trying more risky investments.

Believing advice you get on Yahoo answers can be risky, so read these websites for further information. If you find it too confusing, contact a professional financial advisor. They will charge you significant commissions, however.
Sources:
http://www.vanguard.com/VGApp/hnw/planni...
http://www.fool.com/school.htm
http://sec.gov/investor/pubs/assetalloca...
http://www.diehards.org/readsites.htm
http://finance.yahoo.com/education/begin...
http://finance.yahoo.com/funds/basics

Asset Allocation Calculators
(Determining how much to put in stocks and how much into bonds and money markets is a personal decision depending on your financial status. These Asset Allocation questionaires give you a rough idea how to do this. I like Vanguard best, but try some of the other sites as well.)
https://personal.vanguard.com/VGApp/hnw/...
https://ais2.tiaa-cref.org/cgi-bin/WebOb...
http://www.ifa.com/SurveyNET/index.aspx

Web forum: http://www.diehards.org/
(Many investment web forums are overrun by scam artists. This one seems the most legitimate site.)

You can actually begin before 18 if you have one of your parents sign up for an account for you under the uniform gifts to minors. The minimum investment is $3000. They have many funds to choose from. They choice is yours. Each year end they distribute realized gains so you will have to pay taxes annually on those. For unrealized gains there are no taxes required until you sell your holdings.

Here is a link to Vanguard.

https://flagship.vanguard.com/VGApp/hnw/...