Twitter only allows access to approximately the last 3,200 tweets of an account. If more than 3,200 tweets have been sent, only the first possible tweets can be shown. The information is only provided for unprotected accounts.
Please note that with the introduction of API version 1.1 there is no shortcut anymore to get to your oldest tweets quickly. If you have sent a lot of tweets, it will require some time to process the data!