When I ask myself why someone as distinctive and on target as Ron Paul is even allowed on a special interest controlled media debates, my answer to myself is consistently the same: because he is too old to be a threat. I don't mean to disrespect the only man on television I admire, but I believe revealing the truth is more important then hurting anyone's feelings.
To some extent I do agree that his age is a factor. However, I think Ron Paul is allowed the media coverage he's allowed in this year's compaign largely because he has been a figure in politics for so long and is a legitimate candidate with a legitimate following and therefore can't be entirely denied access without his absence being very noticeable. You can observe, however, that the media, especially the most conservative networks, attempt to discredit him (at times very blatantly) or more commonly simply deny the existence of his role in the debates. This is because he is such a distinctive and on target character, as you said, and I do believe that even with his age he poses a real threat to the other potential candidates.