This will depend on how your account is configured somewhat. Assuming the instance is using the defaults, then any old topic you read will start tracking the read state after 4 minutes. You can change that in your preferences so it happens sooner.
I set most instances I’m on to track as soon as I open the topic.
Now, topics created prior to your account’s creation date will not show up on the new or unread tabs until you’ve actually viewed and started tracking the topic.
You can always find them in the main Latest tab and the per-category Latest tab.
On some instances you may see an Unseen tab, and this does something to similar to Latest except that it is filtered to show any topic that has had a new message posted since your account was created even if it’s not on your tracking/watching lists.
Not all instances add to the menu, but if you’re using a regular browser you can manually access it via /unseen
For example, it’s not enabled here, but going to HTTPS://meta.discourse.org/unseen will show it 