You can deliver an enthusiastic yes, or you can explain why you’re not the right person for the role but can help the young person think it through and find someone who is a good match (assuming you can actually provide this sort of assistance). There is also a third option that allows you to treat the request with the seriousness it deserves. If, whether for personal or professional reasons, you just can’t serve as a mentor or offer the young person help in finding one, be direct and explain why in as much detail as the situation allows. In this circumstance, as in many others, a short no is better than a long maybe.
Sometimes the best mentor is one young people already know and have recruited. There is growing evidence to suggest that this approach can be even more effective than formal mentoring.[faq category=83 template=accordion]
Once you do and have made an assessment of the obligations such a role will entail and whether you can fulfill them, you have two equally good options.
Your first response should be to say that you are honored. Treat the request with the responsiveness and respect it deserves, letting the young person know that you are pleased to be asked and want to know more about what he or she has in mind.