Then how, you might ask, is Purple Tentacle not dead, since we can see his tombstone and all. Well, you'll just have to tune in this Friday to find out. Same bat-time, same bat-channel.
Oh, and about using 1987 as Purple's birth-year; that's the year that Maniac Mansion came out. I'm pretty sure his age is never mentioned in that game, and I know for a fact that it's not mentioned in DOTT, so 1987 seems like a reasonable estimate.
If you've got any questions or comments please don't hesitate to email me.