If you do that with no instructions and education at all, there is a high chance to lost it or someone around her steal it or throw it away without knowing what it is.
Discussion
I had similar concerns around this as well. But for me to educate my friend on how to use a hardware signing device airgapped with a software solution whilst also backing up the seed phase and then trusting him to uncle Jim it for 18 years for someone else when he is still at the custodial hodling (I’m getting him off the exchanges asap!) level it was getting a bit too much. Maybe in a few years when he is ready we can revisit a stronger strategy! Your concerns are super valid and thanks for contributing :) !
Include a copy of Mastering Bitcoin
The point is that you should not be concerned with education at all. 18 years from now bitcoin is the standard and it will be very easy for her to know what to do. The only instruction is - keep it extremely safe.
Not all countries will adopt a bitcoin standard at the same time and if they do, will they educate their people about bitcoin in the right way with self custody... or will they teach them to buy only regulated ETFs with banks and make bitcoin in self custody illegal for individuals.
Of course, anything can happen. She will have to figure it out 18 years from now, there's nothing we can do.
One potential issue is quantum computers. If that happens, meanwhile, she needs to call for help from someone (I'm assuming we are talking about a child, or she won't study bitcoin in the meantime).