Let's assume that an 18 Years Old(Adult) US Citizen Travels from US To Japan where the age of majority is 20 there ,does the 18 year old US Citizen automatically become minor because the age of majority is 20 there?
Or the same US Citizen travels from a state where the age of majority is 18 to another state where the age of majority is 20?
The general rule is that you are subject to the laws of the jurisdiction that you are in at the moment. When in Rome, obey Roman law. However, if you are 19 and have a contract signed while in the US, going to Japan does not render that contract unenforceable, since the enforcement would be carried out in the US. "Being a minor" does not have a single clear meaning, since for example the drinking age in the US is usually 21 even though the general age of majority is 18. It is generally understood to be the age at which you have legal control over your actions and are not subject to parental / guardian veto. It is thus possible that you would require parental consent to undergo a medical procedure in Japan, if you are under 20.
Supposing you travel to a country where the age of contractual majority is set at 20, then an 18 year old may not be able to rent a hotel room or open a credit card account. Laws do not generally forbid minors from entering into contracts, instead they deem that a contract cannot be enforced against a minor. That would put the credit card company at great risk, and the hotel at some risk, and they may be unwilling to take that risk. As it happens, statutory marriage age in Japan and age of majority are the same, but that is not always the case. So you would not get married in Japan without parental permission until you are both 20. However, despite the age of majority law, you are not a minor traveling alone starting at age 15.
You are subject to the laws of the jurisdiction that you are in.
However, some of the laws of the jurisdiction you reside in or are a citizen of have extra-territorial applicability, so you have to comply with those laws too.