When you reach 18 years of age then you become an adult (used to be 21) and you fend for yourself.
Under the age of 18 you are treated as a juvenile, and parents are still legally responsible for them.
Under the age of 16 you are considered to still be a child.
Argyle take over the responsibility of the parents when they are in their care, and so have that legal responsibility and duty of care thrust upon them.
Duty of care means to safeguard from harm and promote the wellbeing of those in your responsibility. These are specific obligations and legal requirements.
Argyle have deemed that the u18's games are very rarely screened live as part of that duty of care, bearing in mind that the team regularly includes those under 16 in the under 18 teams also.