I don't believe the issue with spoilers is "how
long the series has been out." I think it is how
serious of a spoiler it is. When you're talking
about a character's death or a major plot twist
(like in the Crying Game or the Sixth Sense) I
don't think it is EVER appropriate to blurt it out
in a public place with no consideration for those
who have not seen it yet. 

Now, minor plot points (minor spoiler alert) --
such as in Escaflowne, where Van is revealed to
have wings, big surprise there to everyone, I'm
sure -- are a slightly different matter because
they are not intended to be quite the shock that
major twists are. I'd speak freely of Van having
wings, but would NEVER discuss the Dilandau twist
with others unless I was sure they had seen the
series already.

Hope that makes sense. Your mileage may vary, of
course.