The sims is a life simulation game. So sims will have children, they will grow, go to school, fall in love, have accidents and eventually die. Just like real life.
You can try using cheat commands to keep them alive:
1. Open the console ctrl+shift+c
2. Type 'death.toggle'
I hope that works, I've never personally tried that particular code. You may need to activate 'testingcheats on' first.
Hope this helps
EDIT: sorry, I just remembered you out XBOX ONE as your platform. You won't have the command console available to you in this case. I will leave that solution there for any PC players that have the same problem.
To help you: you can try and use the potion of youth. You need to complete whims to earn satisfaction points and purchase the potion for 1,500 points.
You can view a more in depth guide on Carl's Sims Guide:
http://www.carls-sims-4-guide.com/help/immortal.php