KK,
I understand where you're coming from. If this bloke knocked on my door and he came across as a mature lad then the age thing is irrelevent to the customer. I may not employ him full time but I would allow him to do trial lessons and give him students that had no issue with it all.
If the bloke wants to get his FI rating - let him I say. If he can afford it a that age then he can afford to live on very little too! If he is serious about being an instructor for a career - lets get him hooked now as this industry is crying out for career instructors.
We all had to start somewhere. And admittedly he can't be let loose on everyone - he could well be an asset given time and patience.
I just think if the fella comes across as older - he can pull it off.