The problem with hiring an older coder is the same as the problem as hiring a younger coder: they might not actually be very good.
The thing you need to do with the older coder is make sure that their instincts and domain-specific knowledge of something you're hiring for won't mask a deficiency in general programming and problem-solving skills - like the ability to extensively think through all the consequences of their code changes and decisions, or operate in an unfamiliar environment - which will show up later when the rubber actually hits the road.
With the younger coder, the general-purpose programming skills are probably all they have to show you. This is something you need to address in your interview process: just make sure you're interviewing for what you're actually going to need.
The thing you need to do with the older coder is make sure that their instincts and domain-specific knowledge of something you're hiring for won't mask a deficiency in general programming and problem-solving skills - like the ability to extensively think through all the consequences of their code changes and decisions, or operate in an unfamiliar environment - which will show up later when the rubber actually hits the road.
With the younger coder, the general-purpose programming skills are probably all they have to show you. This is something you need to address in your interview process: just make sure you're interviewing for what you're actually going to need.