You can view a user's older post by searching for their name in the search bar. It will take you to the user post where you can view 350 posts through the different pages.
This will lead to a search result for the user. If a user has a lot of historic posts your finding will be limited to only 350.
If you are looking for older than the 350 available, you have the option of Advance Search.
It will help you narrow down the username, keywords, date range and if you want to focus on posts on all forums or only 1 forum. You also have the option of arranging it from recent to oldest or most relevant.