A mentor should not be a mentee's boss or in his/her direct reporting line.
A mentor should be independent and unable to influence the mentee's performance assessment or career advancement (other than through the advice given to the mentee, of course!). To the latter point, a mentoring relationship will work well only if it is a two-way process - whilst the mentee needs to learn from the experience of the mentor, the mentor needs to get something from the relationship too, otherwise his/her interest will quickly evaporate - this is often learning about a subject from a different perspective, often cross-generational.
In addition, not everyone (even many top executives) makes a good mentor. There are many skills & abilities that a good business mentor needs - key amongst them are the ability to listen & observe; the ability to ask probing, open questions; a degree of imagination (to put oneself in the mentee's shoes to understand his/her perspective); and an openness in discussion, allowing the mentee to lead the agenda, avoiding being too prescriptive and/or 'telling' the mentee what to do.
Also important to note is the timescale and breadth of agenda that good mentoring should address. Contrast this with training & coaching. Training is short-term (equipping an individual with skills to apply to immediate tasks), coaching is medium-term (guiding an individual to achieve a specific goal where the coach has goal-specific experience), mentoring is long-term (helping an individual to explore career (and sometimes life) direction options & choices where the mentor has wide ranging business (and life) experience).