Response: There is, as far as I know, no blanket system-wide prohibition on having people 65+ serve. A person of that age might, however, be excused if either of the following is true:

(1) People are disqualified if they are unable to render satisfactory jury service due to physical or mental infirmity. If the person in question suffers from such an infirmity, regardless of whether it is age-related, he or she can ask the court to excuse him/her from serving.

(2) Each judicial district is required to adopt a plan which specified those it exempts from serving in addition to those which the federal jury service statute exempts. In some districts, for example, the local plan exempts persons over 70 from serving. If the plan in this person's district were to exempt those 65+ from serving, the person could seek to be excused. 

Otherwise, the person would have to show some generic reason, e.g., "undue hardship", for being excused. The person would have to contact his/her court to discuss that possibility. However, as a general matter I discourage people from trying to get out of serving on a grand jury: I think it would be a very interesting experience, and it’s an opportunity to serve the justice system (not to mention one of our obligations as citizens).