You can give it to them when they're older but the sooner you give it to them the better chance they have of gaining immunity. I had several birds last year that were unvaccinated and doing fine. Being a newbie I accepted some birds from a friend and didn't know to quarantine them. Long story short, they had Marek's and gave it to my flock. I lost several, including all of the gifted birds. We read all about the vaccine and decided to inoculate the remaining birds in the hope that the lesions wouldn't grow and cause their death. They've all survived and are healthy girls laying me lots of eggs.
I just got 16 new little baby chicks and they are all inoculated since we know we have Marek's on our property now. You have to weigh whether the risk is great in your area or not. We talked to a guy from the state who told us for our area it's very prevalent and he highly recommended vaccinating all new birds. Of course since we know we've had it it's imperative for us. They are vaccinated with a turkey version of the disease that keeps the lesions from forming (they are tumors that can cause all of the issues). It's not a form that will give the disease to birds who don't have it from what I understand. I don't show but I've read that vaccinated birds are allowed in shows which leads me to believe they aren't major danger to other birds.