My experience shows that older instructors "know what they don't know" (or at least know they dont know it all) and that is what is really important for me. My private pilot instructor was in his 30's, with 4000+ hours, came BACK from airlines because he liked teaching better. My instrument instructor was in his 50's, and does it for fun after work. They were both amazing instructors. They admitted when they didn't know stuff, and worked with me to find out the answer. One prompted me to talk to the FSDO and stuff like that, something I wouldn't have thought of.
Now, the other instructors I've had were in their 20's, 300 hours maybe. They acted like their crap didnt stink. They seemed to think they knew it all. I've had some big arguments with some that resulted in me printing off legal interpretations to "school" them.
When I know more than an instructor, and they don't even act like they need to learn stuff too or at least admit they don't know, I lose all respect for them.