The hard facts of aging may indicate that an older worker can't be as productive due to mental and physical decline, but in not-physically demanding occupations, behaviors will influence productivity more than age. What are those behaviors?
A motivated employee will easily outproduce the clock watcher. Sure, it's the boss's job to motivate, but most missed that class, so it's time to self motivate. Start with the motivating thought that the most productive employees are the winners in the job keepers contest. The trick is this: come to the office and work, not some of the time, all of the time.
Younger employees may be able to reason and move faster, but someone that works their full shift, will outperform one that has no qualms about mixing up the workday with a little harmless social networking, a few personal calls and emails, coming to work tired, and chit chat.
Keep skills fresh. If skills are losing their relevance and value, take the initiative to acquire new ones. Don't be an employee that constantly moans about needing to go to a class, or that doesn't know how to do something, when acquiring skills are keystrokes away. There are millions of self-study courses, professional discussion groups, books and classes that can be used to increase anyone's skill value.
If an employer doesn't have an education reimbursement policy, consider it an investment in future earning capacity. If there isn't time to learn new skills at work, that's what weekends and evenings are for. If those hours are normally reserved for personal or family time, think about the alternative of having too much family time because there is no job to go to.
About those weekends and evenings. If the boss needs help meeting a tight deadline, don't wait for him to ask for volunteers, step to the front of the line. When employees make the boss's life easier, their value skyrockets.
Embrace technology. There is no reason older workers can't be proficient with technology when little children are. Technology today is designed to be intuitive for anyone. The only thing separating a child and an adult on the technology score is desire.