Can you teach an old dog new tricks? Yes, as long as the dog is physically and mentally able to learn, you should be good. The problem occurs when your and the dog’s goals are different! Especially when it comes to the interaction between dogs and kids.
As a professional, I can help people with reactivity (just as I have done with many clients); normally, these clients understand and have the same values and ethics I do. They agree that we will not place a dog in an unfair situation and expect to not only tolerate but enjoy it. That is silly, stupid, unacceptable and dangerous.