@Linkat,
Linkat wrote:click on My Topics and then anything you ever wrote is on there - so you scroll down and on the right hand side on the bottom is a bunch of numbers - select the largest one and voila - you see your oldest post on the very bottom of that page.
Actually "My Topics" is only the topics that you've started. It does not include topics that other people have started, that you've posted on.
It will include your oldest post if your oldest post happened to start a topic. But it will not include your oldest post if that is a post in someone else's topic.
The list is also not arranged in the order of when the topics were started. Rather, it is arranged in the order of when the most recent post in the thread was.
If you go to the last topic in the list and then post on it, you will move that topic to the top of your list. If you delete your post (within the 15 minute window that you are given to do so) that topic will move back to the bottom of your list.
The technique that you describe is a good quick way of finding old topics that you've started however. It's just not necessarily guaranteed to produce your absolute very first post.
It is actually possible to find your very first post. But it is a lot more tedious. On your main profile page there is a list of your 10 most recent posts. You can go back in increments of 10. If you go back a few thousand times you will eventually find your first post.