I surely would put him through the ringer and test his ability, skills, knowledge, etc, prior to signing him off just like you would do to all others. It does not matter if this guy is 30 or 80, he still needs to be within standards to obtain a sign off from a CFI. The FAR's do not give any breaks in what is "required" due to a person's age, color, race, etc prior to a sign off. I would assume he is in relatively good health since he has to have atleast a third class medical that is required for solo flight so this probably would ease some concerns if it were me BUT do not allow him to dictate when/where it is time for the solo/checkride. Your tickets are on the line so be wise and be picky. If you do not like the way that he flies the aircraft then by all means do not sign him off, however if he operates the aircraft safely and he is proficient then don't allow his "age" to be the dictating factor to stop you from signing this person off.
I have trained and flown with a few pilots in their mid to late 70's and they did just fine and passed the ride on the first attempt. One of my former students who sticks out in my mind the most was a retired electrical engineer who owned his own aircraft, was in his late 70's, and had been working on his INST. for a few years and no one would sign him off? Why? Good question but I would assume a few were turned away and concerned about this person's age and health. I was able to get him signed off in less than a few months once he was within PTS and when I felt "comfortable" within him and I had not a doubt in my mind that he would be a safe instrument pilot. This person now is in his mid 80's and he still does fly quite often and we keep in touch somewhat.
Be smart, take your time, and when you feel "comfortable" is when the signoff should happen. Whether the guy is 30 or 80 does not matter, he still needs to meet the same reqts.
good luck,
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