People can change, mature, grow as individuals.
What one did as a youth should not be a chain to bind them for the rest of their lives.
Having listened to the interview he gave, where he attempts to address these issues, I don't really know what to think. He claims to not remember them, which is entirely possible. Although I'm not fully convinced. However he does go on to apologise for anything he did which may have caused harm or offense during that period.
Not being from the US, and having no particular interest in this whole political saga, where digging for dirt seems to be a fairly common practice, I'm prepared to take that at face value. He also says that high school was a long time ago, and makes the fair point that people can change a lot in the interim. I'm prepared to accept that.
I think what matters more are his actions and attitudes now, and the way those are expressed, rather than what happened almost fifty years ago. And, while they can't be forgotten (unless you're Mitt himself, apparently) apologising and acknowledging that, to use his words, "I did some pretty dumb things in high school" show a measure of growth that can't be ignored either.
So, on the subject of whether these specific incidents should disqualify him from holding the office... no, I don't think they should. And I would say the same for anyone else in a similar situation. I have no particular interest in being political about it, since I'm not a US citizen. Whether he is a suitable candidate overall is a different issue, and one I have no real view on.