Can you apply for British citizenship by descent if you are over eighteen and your father is a british citizen and you both have lived in the US since you were born?

Yes, you can apply for British citizenship by descent if your father is a British citizen, regardless of your age. Since both you and your father have lived in the US, you would typically need to provide proof of his citizenship and your relationship to him. Additionally, you must meet any residency requirements or other criteria specified by the UK Home Office. It’s advisable to check the latest guidelines or consult with an Immigration expert for specific advice.