The age of marriage is set at 18 years in the interest of the child's welfare. A premature marriage with the consent of the parents is no longer possible in Germany. Marriages are therefore only possible if both willing to marry are at least 18 years old.
For details, see:
https://www.bundesregierung.de/Content/DE/Artikel/2017/04/2017-04-05-verbot-kinderehen.html