Red Heifer

I was just wondering, I know that the red heifer has to have reached 3 years of age to be sacrificed correct? Can it be sacrificied any time after that, say, even when it gets to 4 or 5 years old, or must it only be sacrificed in it’s 3rd year? I have also read reports over the past few years that there have been red heifers born, but that they have been eliminated as candidates. I was wondering if my information is up-to-date, or if there is in fact an eligible red heifer at the current time?

The Rambam states the heifer may be 3 or four years old. If it is older it is still kasher, but we shouldn't wait lest the hairs turn black. I am unaware of any present candidates.