A mentor is someone who's been around the block and genuinely wants others to learn from both their successes and failures with the hopes of this mentee having a new advantage. He or she is a different breed of individual, one who may be a fierce competitor in one aspect, but a sincere confidant and caretaker in another. Essentially, a mentor is someone who is usually a great people person, a dedicated learner, and a role model for success (however one may define it).
Looking for a mentor should not be that hard of a task is you are a person who can generally collaborate well with near anyone. These are the people who put in the extra work and offer up their services/ideas/advice in a selfless manner. They have learned a great deal in the past and wish to impart some of that to the next generation. However, a great disparity in age isn't necessary when considering to pursue a mentor. They may simply be someone from a different walk of life that you are passionate about pursuing.
The bottom line is a mentor should be someone whose opinion you value, whose advice you trust, and whose plaudits you take to heart. These are the people that make our society great because they leave more than they take and they create much more even after their time is up.