If you wait to cut them, the spikes will form seeds (little balls). If you're cutting for drying, I've found the best time to cut is immediately after the pollen sheds and before the seeds form.

This seems early for spikes to have already formed. If, by chance, your plants were at all rootbound at any time prior to planting out, this could have something to do with the height of the plant and that they area already blooming.

The plant bulks up by growing more blades/stems at the base of the plant (stooling), which results in more spikes. Stooling usually results early on in the growth of the plant and not after spikes form.

Thanks Pudge - that was really helpful. I will cut them soon then, leaving one or two to salvage for seeds for next year's efforts.