Dependant Children joining parents in the UK
Children of British Nationals, EU Citizens or persons with Indefinite Leave to Remain can apply to join their parents in UK on the basis of their relationship. They can apply for Permanent Residence in UK if one of their parents is British Citizen or have ILR. Children of EU citizens can apply for EU Settlement Scheme.
You can apply with the parents if they are coming to UK or you can apply individually if parents are already in UK.
Age under 18 or over 18
Different immigration rules apply if age of the child is under 18 or over and they have formed a family by living an independent family life or if they are applying from inside or outside UK and whether they were granted previous leave as a child of someone settled in UK.