I think as long as they've come into heat once, you're OK. If you don't get them spayed, they start coming into heat more and more frequently, until they're on heat more or less once a week for several days at a time. By that time, you will have dragged them to the vet in desperation at listening to their wailing all night, every night and won't care if it's the recommended age or not. That's my experience, at least.