Optics. It wouldn't necessarily be a bad thing if the candidate himself were younger, but as we all know Trump is going to turn 70 in June. Taking what HRC feels is her due away from her is going to be a nasty fight. There is no way in hell that she will pick Bernie as VP - most likely Castro or another younger person to balance her age.
Trump seems to have no problem surrounding himself with young, smart people. His sons, his daughters, his wife, staffers, his TV spokeswoman, et al. I feel they serve to keep him young thinking and in touch. A VP pick who could do the same would be awesome. Just my 2 cents.
I agree that optics is the main issue. Trump will look too old if he picks a young VP like Rubio.
The VP is not supposed to balance anything. It just gives voters a preview of cabinet appointments. Picking someone who is not qualified is a red flag.
The president is the one being judged. The VP just goes to funerals.