Clearly I don't understand what's going on

So in one of my first posts I described my family. I have 2 younger sisters, 3 older brothers, 2 chilean students, and a mom. It took me awhile to understand the Chilean students but I thought I had everything else figured out. Well.. I was wrong. The two brothers that I have met are named Federico and Nicolas. Federico is 23 and Nicolas is 25. Federico seems to have a job, or at least a part-time job, and has a girlfriend. They use the word polola to describe her instead of novia (polola is a serious girlfriend and novia is a fiancee) but on his facebook it says that they are engaged. The girls are usually a couple years younger than their boyfriends here so I assumed that she was about my age. Nicolas told me that he was in his last year of schooling to become an engineer. I thought that was strange because the program usually finishes when people are 23 so I asked my Chilean friend who is an engineer and he said it's rare for someone to be older than 24. Hmm..
Then I found out that I had them switched. The older one is really the younger one. Federico is 25 and has a job and fiancee that looks older than me and was talking about finding a job and Nicolas is 23 and still a student. And my friend's pointed out that it makes sense because the younger one is a little bit immature about talking to me in ways that I can't understand. So I lived with them for a month and couldn't even figure out how old they were.