US law: what exact time is a person considered legally 18? Say a person was born at noon on January 1st, 2000. Under US law, what is the exact time that person legally turns 18? Is it the midnight before January 1st 2018, or the midnight after, or noon, or something else?
In the US, the common law rule is that a year of age is completed on the day preceding the anniversary of one's birth. Here are some citations from cases that relied on this rule: "Full age in male or female is 21 years, which age is completed on the day preceding the 21st anniversary of a person's birth." State in Interest of FW 130 N.J. Super. 513 (1974) (citing Blackstone) "The common law rule for computing age is that one is deemed to have reached a given age at the earliest moment of the day preceding an anniversary of birth." Fisher v. Smith, 319 F. Supp. 855 (D. Wash. 1970) "The appellant did, then, reach the age of nineteen years on the day before the nineteenth anniversary of his birth" Turnbull v. Bonkowski, 419 F.2d 104 (D. Alaska 1969) This is codified in some statutes and regulations. For example, CFR 404.102: For the purpose of this subpart [...] You reach a particular age on the day before your birthday. For example, if your sixty-second birthday is on July 1, 1979, you became age 62 on June 30, 1979. In Virginia, the Attorney General has confirmed that the common law rule is in effect: a person attains his/her next year of age on the day prior to his/her birthday In your example, barring a modification by state law, or an idiosyncratic treatment under a particular statute, the person would be deemed 18 years of age at the earliest moment of December 31, 2017. For a bit of comparative law, contrast with the treatment in British Columbia: A person reaches a particular age expressed in years at the start of the relevant anniversary of his or her date of birth. (Interpretation Act s. 25)
People aged 19 and 18 are "teens" and legally permitted to perform in pornographic videos. That's how it's legal.
This appears to be a fairly straightforward construction. Section 9(2) and 18b share a number of requirements, so 18b points back instead of repeating them. But not all requirements are shared. In particular, requirement 3 (covering pensions) does not apply to newly-graduated students. That's not strange since students are not paid wages. Also, they're not subject to the 5 year residence permit requirement. As for "sentences 2 to 6 shall apply accordingly.", this covers waivers to these requirements.
I was initially going to vote to close this as a political rather than a legal question, however, I think there is scope for separating out the two dimensions. Our society makes a distinction between children and adults by giving them different legal rights, obligations and protections. If you think about it, there are a lot of things beyond sexual activity where the law does this: voting, drinking, military service, compulsory schooling, legal culpability etc. What things are subject to legal restriction is a political distinction. Now, there is no reason politically why these laws could not have been drafted to say "children can do this, adults can do that" and leave it to the courts to decide who is a "child" and who is an "adult". Biologically and emotionally people reach maturity at different ages so this is perfectly sensible. It might even be more just. However, justice is only one thing that we require of our legal system. Among the others are certainty and efficiency. Providing a bright line based on birth date gives certainty. Not requiring the court to deal with this on a case-by-case basis increases efficiency - justice is not free. This is why jurisdictions use age as a proxy for adulthood. As to why they choose any particular age, that is a political question.
I think it's important to keep in mind the essence of your actions. @Mowzer gave a great answer on the matter in hand, but I'd like to add a point of view. Is it illegal to eat while driving? Like pointed out by @Mowzer, if no law prohibits it, it's allowed by default (lots of countries have this premise). But what about the consequences of your actions? Maybe there's no law against eating in the car, but there sure is one in my country that specific says that you are not allowed to drive without using both hands in the wheel. The only exception to this rule is to take out your hand from the wheel to use the stick to change gears. Another point of view to add is the liability. Let's say you're minding your own business while driving and a lunatic trying to commit suicide jumps in front of your car. It's a pretty straight-forward case, you have no reason to be blamed. Now, let's add to the same scenario your snack; with a small change, you are now facing a accident that may have been caused by reckless driving (another thing commonly illegal). To sum up, the law is not really like math that have axioms that determine true or false statements without any distinguishment. That's why every case is single handled in the law by it's particularities.
This FAQ from the US embassy in Korea explains all of the concerns that a Korean-American might have. If you have a Korean lational parent, you are automatically Korean. Before March 31 of the year you turn 18, you must renounce your Korean citizenship, lest you be then subject to the military service law. If you were a Korean citizen and then gained another citizenship, you automatically lose your Korean citizenship, and if you did not complete your military obligation and did not obtain an overseas travel permit, you may be punished if you ever return to Korea. The travel permit lets you postpone military service until age 37, so if you are living overseas, by January 15 of the year you turn 25 you must apply for a travel permit. The laws are set up so that you must enlist by age 28, but must have obtained a travel permit by age 25. That means if you are abiding by the law, there is no risk of sudden conscription if you're there for a short period (e.g. as a tourist). The risk seems to come from not obtaining a travel permit.
According to German law (Jugendschutzgesetz), §9: Minors 14 years of age and older may drink undistilled alcoholic beverages, such as wine and beer, when accompanied by a Custodial Person, Minors 16 years of age and older may drink undistilled alcoholic beverages, such as wine and beer, without accompaniment, and Adults (18 and older) may drink distilled spirits without restriction as well. Note that the act of drinking itself is not illegal for a minor. It is only the offering or facilitating of alcohol that is illegal (and punishable).
He has to follow the law of the country he is in and those of which he is a citizen. A citizen is subject to their country's jurisdiction wherever they are, however, some laws are only enforced within a nation's boundaries and some have extra-territorial application. 18 U.S. Code § 2423 - Transportation of minors covers the US law (I don't speak Hungarian or Polish so I leave that up to you). It says: (c) Engaging in Illicit Sexual Conduct in Foreign Places.— Any United States citizen or alien admitted for permanent residence who travels in foreign commerce or resides, either temporarily or permanently, in a foreign country, and engages in any illicit sexual conduct with another person shall be fined under this title or imprisoned not more than 30 years, or both. 18 U.S. Code § 2243 - Sexual abuse of a minor or ward gives the age of a minor as anyone under 12 years old and anyone under 16 unless the perpetrator is not more than 4 years older. For your 21+ year old this would be anyone under 16.