Maybe I'll separate magic into categories. 

By the way, available characters are not limited to those trying to escape. You can be brainwashed or otherwise to side with your captives to thwart the escapees or be a guard or scientist who interacts with magic users. I suppose you can have some higher up guy but they are mostly in the shadows so shouldn't be your main character unless they go undercover to interact with us more.

The age groups of the magic users can be adult or younger. Old people's bodies cannot handle the stress unless, perhaps, they've been living with the microorganism their entire life and have build up their compatibility with it to a really high level. There will be two ways to get the microorganism; the first is to surgically insert it at any age or the second is to have it passed down from birth. The second option offers the best chance at a high compatibility and many years honing it even further, so is the new preferred method unless they find a highly compatible person that they wish to "hire" into their services. To find compatibility, their body reacts to the organisms swabbed onto their skin like hives. Most of the people given it at birth are around early twenties or younger. That method started only around 30 years ago but wasn't perfected yet so few of them survived into their old age. The early twenties are the first of the experiments to survive that long...that we know of.