You're talking electric?
Something with a teflon/non stick coating that probably can't be easily disassembled for washing?
Does it have a timed or automatic turn off or does it need to be constantly watched to make sure it doesn't burn?
How responsible is this child with other things? For instance do they routinely leave fridge/freezer doors open or cupboard doors open or other doors open when you've asked them not to? If you correct them about something, do they rectify their behavior right away or do they have to be nagged over and over and over?
Have they ever left something electric that potentially could be hazardous to leave on (something that gets very hot for instance) on? How about ever leaving a tap running because they forgot or they got distracted?
In my opinion it's not about the age of the child, it's about the maturity of the child. When I was 12 I'd already completed 4 years of St John ambulance first aid training, I'd completed my Red Cross babysitting course (minimum age to take the class was 12) and was CPR certified. I was running three small businesses in the neighborhood (delivery service for seniors /dogwalking/ cutting grass and shoveling snow) as well as babysitting my own siblings, walking our dogs after school, and doing peer tutoring for classmates.
I definitely would have been considered responsible enough to use a sandwich maker.
My brother however, even at the age of 18, would do things like leave the stove on and walk away, or put something in the microwave, then go fall asleep, then 3 hours later the entire inside of the microwave is soaking wet because it's covered in steam, and that's really bad for the inside of the microwave because it promotes rust.
He'd constantly hit the snooze alarm and then miss the bus to school. He'd use the spare key to get into the house after school and then not put the spare key back in the hiding spot and then I would get home later from school and have no way to get into the house because he was busy napping while the microwave was beeping away full of steaming food..
That kind of thing.
So even at 18, he couldn't be responsible for a sandwich maker. If he used one, and melted cheese or anything else got all over the place, you know he wouldn't have cleaned it, in a timely manner or at all, and then it would have been a giant mess or, you would have turned it on and the remaining food would have been smoking and burning.
So, I don't think any of us can really really tell you how responsible your child is, I think it's a situational question although I'm sure people will have opinions.