Your biggest problem with this would be me.
By the time you get around to mailing or calling them at 67, I've already come and gone at 65.
I could care less at what age they finally retire. I'm on them before they turn 65 regardless. By the time you get there, I've done all the work and they're with me.
Stick with 65
But how do you work this? Say they turn 65 this month, and they dont plan on retiring until 68. How do you work this for the next 3 years? A lot can happen in 3 years. Most people forget what they ate yesterday, what makes you confident they'll remember you next month?