lifting weights by yourself is foolish.
Alas, it seems so. When my son was this age, I bought a BowFlex. I was well-aware of this type of accident. Your kid can swear up and down that s/he won't use the weights w/o a spotter, but shite happens. I thought a BowFlex, with its system of rods rather than weights, was a better choice. Prayers for the family.
Depends on what exercises, how much weight and experience of lifter.
Sounds like all three were a factor here.
Very sad.
Please put a few more qualifiers on that before you sound like a complete 'tard.
There is absolutely nothing wrong with lifting single hand dumbbells and lifting using machine weights by yourself.