They were considered an adult by the time they were 12.
Forget high school prom or working to make it into a decent university to get a degree in a field that will have nothing to do with the career you wind up in. By the age any of that shit would have happened for a Viking kid (you know, if it had existed), they would have already been married and popped out a few Viking runts of their own. Although, considering the average lifespan was around 50 years, getting married at 12 would have been like walking down the aisle at 25 today, so, perspective?