If he's too old, I reject the offer. No use for old geezers without proper armour and virility.
Otherwise, I take nearly everyone - give him a tutor, put him in a city with good educational system, let him stew there for 10 years ...very often I get a quite decent governor, sometimes even a general. If he still remains a complete failure, he might be used for border patrol duties including the toils of building watchtowers and forts or for banzai-charges against rebel stacks. (Yes, I like role-playing with FMs). I think it is quite nice to have a stack of replacement rulers in case the older generation kicks the bucket en masse. Plus I get the feeling that the requests for marriage drop significantly if you reject suitors too often. But when I can afford it, i.e. I have enough pure-blooded FMs, I usually reject only the first offer, if the guy is a complete loser, and normally, the next suitor is of better quality. But I NEVER NEVER accept anyone who is disloyal: Might be an enemy in disguise...
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