Hi,

Its not just the age of the bulb, it is also the size. Usually mine flower when then are about 3 or 4 inches in diameter. The bulbs grow bigger with time, to some extent, but also with proper care. They need to be watered and fertilized regularly throughout the growing season to increase in size. When I dig mine up in the fall (they spend the summer outside in a shady section of the garden) I don't cut off the foliage but let it die back naturally. This allows some more nutrients and photosynthate to go back to the plant. You can just leave the bulbils attached to the mother bulb. Often if they are too small, they tend to desiccate during their dry dormancy. That may also prevent them from flowering. Eventually they will bloom.