Some issues around “underage” sex can still apply to young people who are aged 16 or 17.
It is a Child Protection issue if anyone aged 16 or 17 is involved in any sexual act (kissing, sexual touching, oral, anal or vaginal sex) with anyone who is in a position of trust in relation to them. It is also a criminal offence for the person who is in the position of trust. This could be a teacher, sports coach, someone who looks after the young person in a residential unit, a driving instructor etc.
The person in the position of trust has to be in a position of trust for that young person. For example, a driving instructor having sex with one of his/her own pupils aged 17 is committing a criminal offence, and there are child protection issues for the pupil. If, on the other hand, a young person is having sex with someone who happens to be a driving instructor but is not their own driving instructor, that is not a child protection issue or a criminal offence, provided the young person has capacity to consent and consents.
In the case of teachers, the member of staff would have to be teaching in school the young person attends or coming into contact with them in their professional capacity in some other way. The same principle applies to young people aged 16 and 17 in residential settings.
Where a young person is aged 16 or 17 and lacks the capacity to give consent through disability or ill-health any sexual contact would be regarded as requiring a Child Protection response. The person having sex with them would also be committing a criminal offence.
The situation is exactly the same when someone is 18 or over and lacks capacity to give consent to sex with the exception that their protection would be dealt with through guidance for Adult Support and Protection.