Not every life is meant to be lived until we are old and gray. And not all accidents that happen on Earth are accidental. Sometimes, a person (or yes, even an entire group) is only meant to live until they are 20, or 5, or 2, or 50, etc. Everyone's life span will be different and there's not much we can do about it.
We do have some influence on our longevity. How we care for ourselves during our life on Earth does play a part, just not as much as we would like to believe. I do understand this will be a hard pill to swallow, but we do not have as much control over our lives as we would like to think we do. And, sometimes, this belief can hurt more than it benefits us.
Loss can be hard. Especially when it was sudden or if the person you lost was very young. And as hard as this will be to hear, there was a reason for it. I cannot give you what the actual reasons could be without violating the rules of Earth. However, what I can do is give you one of my theories about why someone's life was as short as it was. No, it won't take away the pain or bring that person back. This is only meant to help bring you or whoever is going through a time like this some acceptance of the situation, whatever it may be, so you can move on and continue with the rest of your life.
So why would someone be born to a life on Earth and only live a few short years? My theory is that they were not born here to learn anything and their life was meant to teach the people around them instead. Perhaps the people surrounding them needed a reminder of how precious time is and how none of us know how long we are destined to be here. How we need to stop wasting whatever time we have left on money or those mundane tasks we often fill our time with. And how important it is to spend time with the people we love and do those things that truly make us happy. Whenever I hear stories from someone's short-lived life they are often filled with tales of how smart and wise beyond their years that person was. Makes you stop and think for a moment, doesn't it? There was a good reason that person was brought into your life the way they were. These people's lives, although short, always leave their mark, and that was the intention.