You can vaccinate any bird for Mareks at any age, but you always have to remember that the older the bird is, less of a chance the vaccine will be effective. You can divide the vaccine up into 1/4s to make it go longer (before you mix the tab and the dilutent). I do not vaccinate my chicks at day old either. I keep my birds in the brooder in the house away from other birds straight out of the incubator. I wait till I have several babies before I vacicnate and do them all at once. It works out well and have not had any of thos ebirds come down with Mareks. I know there is always a risk, but so far so good.