When your still on the boards after you have graduated from high school
When you spend the summer before your freshman year in college in a research lab and being overly excited to finally run PCR and gel electrophoresis.
When you visit the SO camp ( when you only really know the instructors there) during breaks of your research program.
When you join an SO club in college
When you can still name almost all the trees you walk past...3 or 4 years after you have done the event
When you have posted in at least two older versions of this thread
When you know in a few years that someone will write "when you remember the old forum" ( not quite a been in SO too long thing, but it will happen)
When the majority of your conversations with friends take place on various scioly boards
When this thread still comes up with new comments after years of going on
When over 50% of the posts not posted by you (and 100% of the ones posted by you) can be applicable to you in this thread
When your post gets exceedingly long and you realize you could put more. (But I'm willing to share

, someone else put up a list)