If you can demonstrate, by means of certain evidence specified by the immigration rules, he is still dependent on you and not leading an independent life, he will be eligible to extend his stay as your dependent despite being over the age of 18. He would also be able to apply for indefinite leave to remain with you after five years in the U.K. (provided both parents are eligible, or one parent can show "sole responsibility").
How can my son extend his tier 1 investor visa when he reaches over 18 years old?
We applied for a U.K. tier 1 investor visa when my son was 17 years old. There was no problem including him in our application. But now our initial visas are about to expire and we need to apply for an extension. Our son is currently 20 years old. Is he also eligible for the extension? And the next time his visa expires in two years, would he be eligible for another extension?
A child who is already in the U.K. as a dependent of a PBS migrant can extend his/her leave and further apply for indefinite leave to remain in line with the main applicant, even if the child is more than 18 years old. However, he/she needs to meet other requirements in order to continue qualifying as a dependent. These include not forming an independent family unit and not leading an independent life. Additional evidence is also required to show that the child meets these requirements.