I started filling out the paperwork a few years ago, but got stuck on the question "Why do you want to be a DPE?" I couldn't think of an answer that I thought the FAA would find acceptable.
Aren't the mins 2000 hours PIC?
There's a written you have to take and a training session you have to attend. And then you go on a waiting list. When a need comes up, the FSDO requests the top 3 candidates and they choose. I'm a bit fuzzy on the order of events, so don't hold me to the above.
The very youngest DPE I've ever seen got the designation when he was about 40 years old; I suspect they like the gray hairs.