grumpywolfman
Black Belt
Knowledge and teaching ability would be the most important to me. But I suppose the first questions that would cross my mind would be: can he prove his lineage, and does he know how to throw his weight into his punch? 

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Might make a difference if you're talking about a "Once Fit & now older & overweight" ... or "Never Fit."
Anyone remember TV's Kunf-Fu Series? ... Master Po was a bit Fat, Old, and BLIND! Yet none of that negated the wisdom he had to share..
While I like the sentiment, I've seen pictures of him and while he's not in the same shape he was (who is?) he doesn't look particularly old or fat, especially considering that he's 54. Recent pictures I've seen of him indicate that he is no fatter than Liam Neeson was at the same age. I haven't seen any shirtless shots, so couldn't tell you if he's maintained a fairly ripped physique, but he certainly didn't look fat or out of shape to me.It's not JKD, but have you seen Jeff Speakman lately? Far from his "Perfect Weapon" physique, much closer to the 'old fat guy' build today, would you refuse to train with him? If I lived in Las Vegas I'd have no problem learning from him.
He definitely looks a bit more portly than pictures that I have seen, but given his age and credentials, it wouldn't put me off.Maybe he's lost weight in the last few years, but the videos I saw of him from around 2005-2007 did not show him in great shape, even for being close to 50 - neither am I, I certainly can't say "look at me, everyone should be in this shape" (which is more round than ripped these days).