I don't totally buy into the "dan grades are time served grades." One needs to progress in technique along with wisdom and experience. There are physical limitations that age places on the body, but being in general good health one needs to work hard just like the kyus.
This actually hits a little nerve with me. I know that my approach in training for nidan wasn't even close to what it was for shodan.I could see it as I wasn't pushing my self as hard as I could have.I was young enough that I should have kept up the intensity of my training.My fault, and since starting up in another art have I'm careful to not cheat myself in my training. It also is great that I have a sensei that tests his students when he thinks they are ready.There isn't any time table.