How old must your child be to join you in the UK?
As a general rule, your child must be under the age of 18 on the date of application.
But your child may be aged over 18 on the date of application where the child was last granted permission as your dependent child, and seeks permission to remain in the UK as your dependent child.
It follows that your child aged over 18 cannot get permission to enter the UK for the first time as your dependent child.
If your child is aged under 18 on the date of application, there must be suitable arrangements for the child’s care and accommodation in the UK, which must comply with relevant UK legislation and regulations.
If the child is aged 16 or over at the date of application, the child must not be leading an independent life. What does it mean that the child must not be leading an independent life? The Asylum and Immigration Tribunal’s decision in the case NM (“leading an independent life”) Zimbabwe  UKAIT 00051 indicates that the child “must not have formed through choice a separate (and therefore independent) social unit from his parents’ family unit whether alone or with others. A child who, for example, chooses to live away from home may be leading an independent life despite some continuing financial and/or emotional dependence upon his parents”.
If you want me to assist your dependent child in the process of applying for permission to enter or stay in the UK, feel free to contact me. Please note that I am an accredited immigration adviser, not an employee of UK Visas and Immigration, and that I charge fees for the advice provided.