In my opinion, the data showed that voting on new questions and answers was good (2 votes per post). Though you're right that voting on late answers is poor, but I think that's more to do with factors other than simple voting.
Abandoned Questions
Many times somebody will drop in and ask a question, though they often will not have supplied all the details required to answer the question. In this case, the community will use comments to try and get more details. Unfortunately, this effort is often in vain and the asker has moved on, never to return.
Some folks will throw out answers to these questions, hoping to get a few up votes. However, due to the lack of detail in the question, it's really impossible to provide an accurate answer. It seems voters are able to recognize this, and won't cast votes on the "stab-in-the-dark" answers.
A Vote To Close - unclear what you're asking, is probably the appropriate response when you come across these questions.
Asking the wrong question
Some folks will try to ask a question, but they'll ask it in a strange way, or ask the wrong question trying to beat around the bush. This is very often the case where a user wants to ask an off topic question, but they know it's off topic. Instead of not asking at all, they try to make the question seem on topic. Or the user really doesn't know what to ask, but they need help so they give it a try. Again, the community will use comments to try and get at the heart of the problem. All too often though, both parties get frustrated and stop trying.
Once again you'll probably get some "stab-in-the-dark" answers, but it's too late the question is lost. If you can get lucky, and your "stab-in-the-dark" answer hits the mark, the OP may return to reward you with a check mark. You may even get a few up votes, though don't expect a landslide of votes.
Buried by Crap
Every once in a while (usually on weekends, when nobody is looking), a flood of questions will come in. Sometimes a good question (asked early), will be buried by a bunch of other questions. These are good questions, that can be answered, but they slipped by without being noticed.
If you can find one of these diamonds in the rough, you could end up with a check mark and a bunch of up votes. Though be warned, this is a rare case.
Requires too much technical knowledge
Sometimes questions are asked, that can only be answered by a user with lots of experience in that specific area. All other users just won't have the knowledge to provide a correct answer. If a person with said knowledge does not see the question, it too can be lost to the abyss.
In this case, even if it's a good, popular question, and you provide a great answer. It's not likely to get many votes, since the general population really doesn't know what the answer is.
In the end, if you're looking to gain reputation on the site. Answering old questions is not the way to do it. You could try the shotgun approach, and blast a bunch of questions with so-so answers. Get lucky and provide a decent answer to a really easy, really popular question, and get hundreds of up votes. Or you can slog along, providing good answers, and slowly build up your reputation within the community.
Just remember who your audience is. You're not answering questions for professionals in the field, you're talking to people who don't work in the industry, and are only doing the job themselves because they can't afford to pay for a professional (I'm speaking in generalities, of course. We're not all poor, inexperienced desk jockeys). The real challenge is explaining highly technical information, that requires years of studying to understand, to the average everyday person. If you can do that effectively, you'll get lots of up votes.