I'm 26 and I've had plans to be cremated for a few years now. I've named one person and an alternate (in case the original person is unable) to spread my ashes over my favorite forest from a natural bridge nearby. I've also made arrangements for the care of my children, and I have a general will for my material things, along with a living will, and an organ donor card.
I did all this when my kids were born, and I review and update it once a year. Call me paranoid, but I don't like leaving things up to chance.