When does an academic recommendation become meaningless? By the time I apply the academic referees whom I intend to ask will not have taught me for some six years.

I understand that professional recommendations are preferable and I do have one in mind but Harvard and Stanford both require three recommendations so I was leaning towards asking a professor for at least one of them. Is this inadvisable? Would it be better to ask someone who might not know me nearly as well but has interacted with me much more recently?

Lastly, are academic recommendations in and of themselves anathema or would one be acceptable from a professor who taught me within a year before applying? (University course was taken for enrichment purposes.) Are such recommendations typically avoided simply b/c they tend to be old or is there something that inherently makes them undesirable?

Thank you very much!