12 year old's do fall into the high risk category for cooking, due to there developing bodies and immune systems. Also pregnant women and elderly people fall into this same category and is highly recommended that these people have beef served 140 degrees F or above for safety concerns. Typically it has been agreed that meat at 120 degrees F maintained at that temp for 10 seconds should kill most if not all the bacteria (please keep in mind that this is for cooking beef ready to be served and not holding temps). However variations of bacteria can mutate their tolerance to heat. As far as USDA is concerned, it is NOT acceptable to serve children beef below 140 degrees F.
For me with this knowledge and cooking for many years, I always will let the person know the risks of what could happen and discuss the painful bacteria that could make them sick if eating beef under 140 degrees F for the high risk population category. But I will cook it to the temperature they prefer, after all they know there bodies better than I do.
I do NOT consider hamburger within these guidelines because, hamburger is made from the most outer part of the cow flesh which has a higher potential to be exposed to fecal matter at the slaughter house. Hamburger for high risk population should be cooked 160 degrees F or above for safety concerns for the high risk population category.
This information was ingrained into me with ServSafe, any certified food manager at any restaurant knows these practices and should let the customer know the risks.
In the chart below you removed them at the low end temp range and let them rest out towards the high end range to get that perfect doneness (and yes, 5 degrees makes a big difference).
Hope this covers it