In June 1999, while Governor of Texas, Bush announced his candidacy for President of the United State
June of the year prior to a presidential election is when many major candidates announce formally with a big campaign event. They form “exploratory” committees prior to then for the purpose of putting together an organization and fund-raising apparatus, lining up key supporters in primary states, getting a ground game up and running, and so on. All of that needs to be in place by the time of the formal announcement/kick-off event so that the actual campaign can get off to a running start.
Candidates like Pawlenty, who lack national name recognition, start the process earlier precisely to raise awareness. Candidates with name recognition can start a little later, but not much, because key people in the states will have already committed to other campaigns.
Just want to add that Palin has done none of the things that serious candidates do in these early months. That’s why savvy political observers do not believe she is running. Might she decide to get in after all? Yes, but the longer she goes without doing all the necessary preparatory work, the harder it will be for her to overcome the challenges of organizing a national campaign apparatus.