"If a man has a stubborn and rebellious son who will not obey the voice of his father or the voice of his mother, and who, when they have chastened him, will not heed them, 19 then his father and his mother shall take hold of him and bring him out to the elders of his city, to the gate of his city. 20 And they shall say to the elders of his city, ‘This son of ours is stubborn and rebellious; he will not obey our voice; he is a glutton and a drunkard.’ 21 Then all the men of his city shall stone him to death with stones; so you shall put away the evil from among you, and all Israel shall hear and fear."
These verses refer to that time somewhere between age 16-21, where the child is still under parental authority, but of sufficient maturity to be willfully, persistently, long-term stubborn, and know most of the ramifications of it. An 18-19 year old doesn't have to "grow out of" their adolescent rebellion before excommunicating them. At that age, they know exactly what they are doing and are guilty.
Then, as now, there is a court of appeal beyond the parents. If a 17-19 year old son is rebellious (for instance, physically harmful or endangering of his siblings or mother, and father is absent or doesn't restrain him), this verse applies. I would say that it applies today not in that he should be executed, but that he should be turned over to the civil authorities and/or the church.
Once the rebellious young one is not under parental authority, this verse inherently no longer applies, as the elders at the gate can bring him before them himself. "He is of age; ask him." So this verse doesn't speak to the assertion that a son is under his parents' authority until married.
There is no verse that says an adult has to obey his parents. We ought not misapply verses that appear to say it, just to have absolute clarity on how it goes in that 18-25 year old (or beyond) phase of life before marriage. We ought to be releasing and launching them in that phase, rather than overly directing and controlling, which squelches the growth of adult-like decision making, which they need to be mastering at that age. That they can practice with parental oversight/protection is a gift.
Notice there is no punishment or guilt implied or imputed to the parents in this scenario in Deut 21:18-21! A parent can do all in his power to restrain and disciple his children, and still end up in this situation. If not, this verse wouldn't be here. This does not undermine the truth that we believe God for the salvation of our children, or that parents are covenantally responsible for the sins of their children. Deut 21:18-21 is how they carry out their responsibility with a child who is out of control: they take him to the church and/or state.