Ask an Immigration Solicitor. Get an Answer ASAP.
I am a solicitor.
Unfortunately, the cut off age is 18 years for children applying outside the UK.
You may include your son in your application, include all supporting evidence of his dependency on you and see if the UK Home Office exercise their discretion and grant him a visa.
May I help further?
Successful visa applications,including yours, cannot be guaranteed and in your case, your son does not count as a child as he is outside the UK and not under 18. Whether or not he is a dependent, that does not matter I am afraid which is why you would be relying on discretion and you will only know the outcome if you apply.
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I trust that my answers were useful.
All the best