I have my 8th grade Robotics 2 students using a soldering iron. The first soldering project is a little LED christmas tree. For most of them it is the first time they have ever soldered.
A few weeks ago I was helping a student soldering some leads on to an eight way rotary switch. I was holding some wires for him and told him to be careful not to burn me. Of course he did. It was nothing bad. In fact I was able to say, "look at what the iron is touching."
I do instruct them that there is no food or drink between soldering and washing hands.
Obviously, it isn't the main part of the class. The core of the class is an introduction to electronics and programming with the Arduino, soldering is just a small part of it. This is after the Robotics 1 class.
Robotics 1 is generally 6th or 7th grade. The core of that class is a set of lessons that have them using the Lego EV3 robot. One thing I find a bit odd is that I am using the same robots and textbook that is used at the local community college in its introduction to robotics course. Of course, I also use the same book, and lessons in my 7th and 8th grade introduction to Photoshop class as the community college uses.
Back to soldering, I generally start it at 8th grade. However, in a small number of cases, I have gone down to 6th grade (I am thinking of a particularly apt 6th grader who started in robotics 2, bu that is rare).