Does a tenancy agreement need to be changed when tenant's children become adults, presumably at 18?

Tenancy Types and Management

I have children named in the tenancy agreement but age is not included nor is there any definition of 'child' as if a change is needed if they are already adults or become adult as time goes by so I have assumed they are 'permitted occupiers' who do not personally contribute to the rent and therefore have to follow the tenant in fulfiling the terms of the tenancy agreement so have to leave when the tenancy ends.

I have looked through the NRLA website and cannot find any discussion, comment or advice about this so thought I would raise it to check that my understanding is correct.

Also, I have found one discussion about having an adult join the tenant as a Permitted Occupant acknowledged by a letter from the landlord as an alternative to creating a new tenancy agreement. That does raise the question about the need for a similar letter when or if I learn that a child has become an adult.

Advice will be appreciated.

Thanks