Why do they? The older candidate may have a higher expectation, but as an employer, the compensation for a particular position should be somewhat fixed. If the older candidate doesn't offer anything more than the "inexperienced" one, offer the same salary you would the inexperienced person. They can always say "No".

Trust me: There are plenty of older programmers who do not expect a higher pay because of their age. In my career, whenever I hear "older people expect a higher salary", it has almost always been by employers/managers, not by actual older professionals. On the other hand, I've met quite a few programmers who complained they didn't get an offer or were told they were ruled out because they were "too senior", and the complaint almost always was along the lines of "I was not asked salary expectations".

Not as differently as you might think. When I was more junior, I assumed the wage increases would continue as they had been. But they flatten out after 10-15 years of experience, to the point that the salary of someone with 15 years of experience is probably the same as someone with 30 years of experience.