I do believe that mistakes in the past should be looked at more as learning issues, and I am tolerant to people's different choices.
However, Let me play devil's advocate, because while some things online will blow over, at a certain point they will make a difference. Also, I definitely see younger people need to learn about TMI when it comes to their online selves...
Drinking, drugs, questionable fashion choices in the form of tattoos and piercings...
Personally, I do not have a problem with any of these. While I have done these in my past, (except the tattoos and piercing) and I still drink (but no longer to excess) They can now and in the future cost you a job; even at a future "young person" company. It is one thing to hire some of these people in the IT industry that most /.'ers are familiar with, but there are only so many jobs there and not everyone can do that work. How many businesses will be willing to hire someone with multiple tattoos that can not be covered up or facial piercings (e.g. nose rings, lip and brow piercing, etc.) Many will need to be in sales, and trust me, even young companies (especially young growing companies) are not willing to alienate their customers especially if they need to sell to the conservative "will somebody think of the children" crowd.
Sometimes they will just be needed to meet with clients. Even as a developer, I have needed to do this... One ex-employer specifically told me (after I was hired) that after technical interviews I was essentially hired, and the face to face interview was only to prove that I "clean cut." If I had tats, noticeable piercing, or even body odor, it would have killed the deal. The fact is if you meet with clients you represent the company, and they want a professional image.
Remember, the professional image is more than skin deep. If a client or customer loses your business card and/or contact information, they may google your name to try to find it. (or may do this just because they can...) If they do not like what they see, they will contact the business owner, and you will be forced to work on a different account, or if there is not enough work, you will be let go. You are only employed to bring value to a company. If you can do this or if others do it much better, do not expect to be employed very long.
some small crimes like shoplifting. On the other hand, serious crimes probably won't be so easily forgiven.
There is a fine line here... Just how many businesses do you think will forgive shoplifting? It is a form of theft, and even though it is one of the most minor form of theft, many businesses will think that if you stole in the past, you may steal again, and they don't want it to happen to them. Recently, there was a local news report about police departments not being able to find qualified candidates. Part of the story specifically mentioned that candidates had to have a clean credit history and that disqualified a large percentage of applicants. While I personally think this is going too far with the recent economic collapse, what other small crimes do you think will stop people from a job?
what happens to the cases where one's old beliefs, ideas and words are brought back years later.
This is very true... Especially if unemployment remains higher than normal, this can always come back to haunt you. Many companies will not give you a reason why they will not hire you. If it is a market favorable to the businesses with many more applicants than positions you will never know why. It could be something you post including religious views, sexism, racism, ageism, a single photo of alcohol, who knows... It may even be a medical condition.(and they don't want to pay the premiums or deal with you missing work.) Many of this is illegal depending on which state you live in, but you will never know what it is and your guess is hardly the proof you need in order to sue a company. (And most smaller companies with under 50 employees are even harder to sue in these cases.) While I think that most younger people (and by extension the companies they found) are more tolerant to others and have fewer prejudices, I doubt it will ever go away completely.
In response to the suggestion that some people are saying young business are more tolerant and will be willing to forgive more in the way of a bad online personality... I say good luck with that. Tumblr, Facebook, and even Microsoft of the 80's, are rare beasts that start small and become instant giants. Most small companies started by young people are small, and stay small for a long time. They scrape by and have a difficult time getting clients and funding. While they may be open-minded, in order to stay in business, they need to listen to the few clients they have... If their clients are not as open-minded, expect those attitudes to trickle down.