Charles Eugene, Duke of Württemberg was declared to be of full age ahead of time in 1744. Source: Internet
In 1893, King Alexander, aged sixteen, arbitrarily proclaimed himself of full age, dismissed the regents and their government, and took the royal authority into his own hands. Source: Internet
People sometimes reduce their age on marriage, and those under "full age" may increase their age in order to marry or to join the armed forces. Source: Internet