For age I think really anything over 60. Mainly because your body doesn't regenerate as well. I'm saying 60 because I know of people who did it between 40-55 and had good recoveries.
For starting height, I'd say no. I believe that everyone has the freedom to what they want with their bodies. So if a 5'10+ guy wants to take a risk with such an invasive and risky surgery that has yet to yield substantial results in multiple cases, then by all means go ahead prove how crazed you are.
I think mental state is one which the doctor should be really restrictive with. And with this in mind, mental state plays a big role with starting height. Example: If you have a guy who's 5'10+, in say the U.S, and wants to go do LL, then maybe the doctor should ask for a psychologically evaluation from the patient. Because that seems like he's out of touch in some regard and is simply creating despair in his own mind. But in truth, there should be a sort of evaluation from all patients regardless of starting height to make sure they are in a right sort of mindset.
Now keep in mind one extremely important factor that pretty much holds all my points together in regards to this response: LL isn't like any other cosmetic surgeries: it's a sort of painful, arduous, journey. One little fk up in that journey will destroy the life you once had.