Student or Learner
"A thirteen year old person is themselves a child. They cannot babysit an infant."
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
For me, themself is more logical than themselves in this sentence, where the subject is logically singular.
- A thirteen-year-old is themself a child.
- Thirteen-year-olds are themselves children.
I do think the latter is the more natural phrasing.