It's actually deliberate on that vandal's part. He picks something that he thinks will make other people upset and then just repeats it over and over again until he decides to pick something else. The key is to remember that this person is looking for attention and wants people to get upset by what he's doing because it gives him attention. I've seen him spend an hour vandalizing a wiki, but it takes me as little as 15 seconds to clean it up.
That's okay. It only takes me a couple minutes to sort through the changes and get everything put back the way it should. In this case, it took them a half hour to commit the vandalism and I spent about ten fixing it because I was taking my time. Otherwise, I might have had it done in about three. That's about the way it's been going. It's usually ten times easier for me to fix the vandalism than it is for them to commit it.
It depends. My birthday is a few months after the school year starts, so I was still four years old when I went into Kindergarten. If I remember right, there was an assessment to make sure I had learned certain things before I was allowed to join the class. So at the beginning of the fifth grade, I was 9 years old.
Looking at it from the other angle, what if Lincoln was held back a year for any of the classes from Kindergarten all the way up to the fifth grade? If that happened, he could very well be 11 years old and still in the fifth grade. It may not have happened, or it could have and the people making the show haven't felt the need to say it.
My advice is don't get hung up on trying to prove you're right. For most cartoons, it isn't important to the stories they're telling to give characters specific ages. Unless it's something special like an episode that takes place on a birthday, a lot of them just imply that characters are around a certain age by how they look and how tall they are, etc.
Going along with that, try to avoid what other people do, which is to assume that a new season of episode means that a year has passed, so therefore, the characters are another year older. Time is different for cartoon characters. To give you an example, Phineas and Ferb ran 4 years. All of these episodes take place during the summer of one year when the kids were around nine or ten years old. They were never given specific ages or birthdates, and when the show ran longer than the creators had planned on, they had to say that some of the episodes took place in the morning and some in the afternoon of the same day.