Would you judge a person by their past??

Say you met someone new, whether they could be a new friend or a new partner, he/she is a good person, they seem happy and know what they want, you get on well, you like them and they like you.

But say then you found out some stuff about their past when they were younger that would have shocked you. Maybe it could be that they were into drugs and crime so they could relapse and go back to those ways again, or that, if you wanted to date them, they used to be promiscious so you don't know if you could trust them to stay faithful, or maybe they were one of those people who were not popular at school because they would have been considered weird or creepy for whatever reason, it could be that they had some issues or really personal issues like mental illness, and so they would have not much friends.

But would you judge that person on their past even if it was clear that they are so much different now? That they have cleaned up their act and working hard to make themselves a better person. I mean do you really think it is fair to not associate yourself with someone just because of whatever happened in the past? I have known so many people who are so different from when they were their early teens, they have grown up and are working hard to make a good life for themselves than before, when they would have had a lot of issues, more than the typical teenager would have.

Also when I am talking about someones past, I am talking about during their teenage years, since because of puberty, I think it's fair to say everyone has made mistakes at a time where they thought it was completely ok.
I should know, been there, done that. But I would like to think I am different now, only problem is that I am 20 and only left school a year, so any mistakes I made are probably still fresh in people's minds.

So what do you think?