The legal wording can be translated into a birthday.
A child’s first year of life begins at birth. On the first birthday, that year has been completed. In the same way, eighteen years of life have been completed on the eighteenth birthday. This explains the law’s wording without mistakenly adding another year.
Legal adulthood is a legal status. Parental responsibility under section 1626 concerns a minor child; it does not simply continue as authority to direct an adult’s life. Family relationships and support can of course remain important.
The other three answers
The offered numbers fall either before or after the legal threshold.
16
At 16, a person is still a minor. The fact that some rights may exist earlier does not change the start of legal adulthood at 18.
19
Nineteen is one year too late. “Completion of the eighteenth year of life” already means the eighteenth birthday, not the nineteenth.
21
Twenty-one is not today’s general age of majority in Germany. The applicable provision in section 2 of the Civil Code specifies 18.
The age of majority is not the law’s only age threshold.
Age limits for particular activities, protective rules or procedures may differ. “Legally an adult at 18” therefore does not mean that no personal rights exist before then or that every special rule automatically ends afterwards.
Legal adulthood and financial independence are also different things. Whether someone still needs support or has particular entitlements is a separate question from their age under section 2 of the Civil Code.
Section 2: age of majority · Civil Code ↗Eighteenth birthday = legal adulthood.
Think of the actual birthday. Do not confuse it with the calendar year or other statutory age limits.
Three words for this question
- die Volljährigkeitlegal adulthood
- Legal status as an adult; in Germany, from age 18.
- minderjähriga minor
- Not yet a legal adult; in Germany, under 18.
- vollendento complete
- To finish completely; here a particular year of life.