Hola...
I'm from Mexico, so maybe this will help you. In Mexico, 18 years old young men have to make that process im the year he's 18. For example, I started that process when I was 17 on September, but my birthday it's on November. My generation (which is 2003) has to process a pre-card, which is released after one year of military service. And then it's a card. Now, answering your questions:
- If you HAVE to do your service, are there any concrete exceptions (f.e. if you are studying)?
A. Yes. Mexican military service it's most of the times on Saturdays (there are some special national days on weekday so we have to go too). So, if you work or go to school on Saturday, you have to prove it with a certificate, and then the Military Office releases your card, and you wouldn't have to do Military Service anymore. Also, if you have a health issue like Flat feet, vision problems, fatigue, high blood pressure, some bone deformity, etc...
- what will happen if you DON‘T do your service?
A. For being honest, I don't exactly know, because the military law says something but most of the time, it doesn't happen anything. The law says that you could be accused by desertion or not being submissive, which are "hard crimes" and can be punished with jail or penalty fee. But this doesn't happen as I said. What you don't be able to have is your military card, and that's a requirement for some jobs and it always be for government jobs. In my opinion it's important to get it because you won't know when a good job will ask it to you.
- are/ would there be any legal or future problems?
A. No. Fortunately, Mexico is a peaceful county, so government is not severe with it. Currently, military service has been reduced in a lot of things due to the power of the Senate of the Republic. I don't remember it exactly, but I guess the Senate decree that it won't be any legal problem if you don't do your military service. Also, at the military centers, we won't learn how to shoot a weapon (as my dad and uncles did). Military card was requirement to process the passport, but now, it's not.