What does this quote mean?

Basically you have to understand the times in which people lived before you judge them harshly. If you were a kid who grew up during the age of slavery then you wouldn't see anything wrong with it, it just was the way it was. Today people have a different attitude towards slavery.

I think that it means you have to understand a person to judge them.

'By the standard of their own age' I think means maturity. Like, you can't judge a teenager by comparing them to an adult. The same with comparing a little kid to a teenager.

If you are going to judge a person, look at their peers first. Then you can get a better look on how that person feels, thinks, and why they do the things they do.

People must be judged by how other people of the person's age act.