I have a teenage daughter, she is 13. She is in the process of learning.
I don't allow her to cast any spells alone. When we do cast together it is very simple and easy "wishing" moon spells. Before we get to the full moon, I have her write out her wish and tell me why she wants it.
We do this because she is still very young, even though she has a high IQ, I want to make sure she fully understands what she is asking for and if it affects anyone else.
Teenagers are at an age where instant gratification tends to win out over logic and forgetfulness is a common theme. These are very dangerous elements to bring to magick.
When you factor in puberty and maturity level, it can cause chaotic energy and magick. Imagine what gets sent out if you are angry and can't control it, but your friend asks you to do a healing spell for her. You're not meaning to do harm and you're not angry at your friend, but your emotional and even though you are trying to do a good thing, it comes out all wrong and could end up harming someone and yourself.
A mature adult (I say mature because being and adult doesn't automatically qualify) would know that they are too upset and that the spell would have to wait until their energy is better.
Also, most adults won't be swayed by peer pressure.
The reason why so much is drilled into young people about magick
Is because it's a huge responsibility.
I hope this helped you understand a little.