The Spathiphyllum seedlings are just full of surprises, it turns out. Only 14 months old, and two of them have decided to bloom already:
Another thing that's surprising about this is that at the moment, some of the peace lily seedlings are in 3-inch (7.5 cm) pots, and some of them are in 4-inch (10 cm) pots, and both of the ones that bloomed are in 3-inch pots.
The conventional wisdom about peace lilies is that they are more likely to bloom if potbound, but I'd always assumed that that was some kind of error, on the grounds that 1) they don't seem to have that much problem blooming if they're not potbound either, assuming that they're mature, healthy plants, and 2) there are no pots in nature, so why should the plants care one way or the other about being potbound. And those still seem like perfectly valid arguments to me, but even so: the ones I left in small pots are blooming, and the ones I moved to bigger pots are not.
So perhaps there's something to the conventional wisdom after all?