Why a 40th?

As somebody noted to me last week, the 40th reunion is kind of an odd one. The 10th, 25th and 50th are obvious ones (are there others?), but the 40th just doesn't seem to fit a numerical pattern. Perhaps the organizers, or someone who has been to previous reunions can answer better, but it seems to me to fall to one of two reasons:

- 25 years without is a pretty long stretch, and/or

- At this age we're passing away more rapidly (per a WSJ article yesterday); there will probably be a lot fewer of us 10 years hence.