Google has filter in place for new websites. During this "sandbox" period they do not use your backlinks which calculate your search engine rankings (backlinks still show, PR shows). Since backlinks are very important in the google algorithim, it makes it extremely hard for a new website to get any google search engine traffic for a competitive keyword when they do not factor in your backlinks. This filter lasts at least 3 months, and some claim up to a year (depends on the keyword). Once google releases your site from the "sandbox", your rankings can see a huge jump overnight. There is nothing you can do to prevent or get out of the sandbox - as it based on the age of your website.
Google grandfathered older domain names from the sandbox as long as ownership doesn't change. If you buy an older domain name from someone else, it will be treated as a new name (subject to the sandbox). This was done to prevent someone from just buying an old domain name to escape the sandbox.
This is for domain names only- established/developed websites are exempt from going back to the sandbox i.e. if you buy an established 2 year old site, it isn't going to be sandboxed when the domain changes ownership.