How do rescue dogs get trained?

SAR dogs are trained from being only a few weeks old, and this is done by a professional.

2nd Answer:

You don't have to be a professional to train your dog to be a Certified Search & Rescue dog. You must know the way to correctly train your dog to track. You can get books on this or find a dog club that teaches tracking, etc. Training your dog to track is one of the finest things you can do as a team. You should start them out young but not always necessary.

I started out one of my male bullmastiffs at the age of 3 years. We did have help from the owner of the certified SAR Bloodhound who was then the only certified Search & Rescue dog in Siskiyou county. The bloodhound was dying from cancer and we slowly took over. So this does show you that you don't have to start out super young, it is best, but not necessary and you can do it yourself with the proper guidance & training.

Certification is now under the umbrella of the Dept. of Homeland Security with testing often done at a reasonably close U.S.A.F. base. You can google the requirements.