So can I vaccinate all my older birds?
Chicks should be vaccinated within 36 hours of hatching and then be quarantined for 3 weeks. People do vaccinate after this time but there is no evidence to suggest that this gives any protection. It doesn't stop them from contracting the virus it merely helps to minimize the symptoms/ tumours.
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq