Can I appoint a minor (underage) to be my executor / guardian?
Executor(s) / Guardian(s) / Trustee(s) must be above 21 at the point of acting.
However, you can still appoint someone underage (< 21) to be your executor / guardian / trustee. If so, you might want to consider adding another adult as a substitute. You can do so by clicking on the (+) button in the respective sections.
This way, in the case where you die before your "main" executor / guardian / trustee reaches 21, your substitute(s) will kick in.
However, if the Will / Trust kicks into effect before the children comes of age, they will not meet the criteria to become one, and the substitutes (if any) will act instead. This is also why (while not mandatory) it's generally good practice to have at least one adult as a substitute just in case.
In the event you live a ripe long life and your "main" executor / guardian / trustee reaches 21, then they will be appointed accordingly.