Instructors often invite colleagues from outside the university to speak to their students. These presentations by practitioners typically include a review of the speaker's career and thoughts related to the company, industry, and/or function he or she represents.
Students gain valuable real-world perspectives from these speakers that energizes what they've absorbed from assigned readings. Guests also may direct students to internships and jobs in the field.
Yet I've seen guest speakers misused in courses in two ways.
First, presentations must fit into the overall learning objectives for the specific class. A guest speaking on any topic generally related to the course might offer value, yet without an explicit connection to the particular session's learning outcomes students may not reap all potential benefits. The relationship between theory and practice is critical, and it's incumbent on instructors to skillfully incorporate guest presentations only when there's a clear fit with content.
In my experience, it's harder to plan other activities around an outside speaker than it would be to speak or facilitate activities yourself. That's another reason why I recommend guest presentations only after carefully planning the entire course.
Second, I've learned that not every guest speaker connects equally well with university students. That might be because of their public speaking skills, personality, and/or experience in the field. Unless you know someone well it's sometimes hard to tell. Their age doesn't necessarily make a difference. I recommend that the instructor speaks with the prospective guest ahead of time to discuss the topic and offer suggestions on how they can best address student interests.
In short, students gain most from skilled professionals addressing issues directly relevant to what they're learning in class. And those professionals reap greater satisfaction when the instructor spends the time to ensure a fit between the topic, individual, and the students.