3 sentences

1. 年满十八岁的人才是合格的选民。

   nián​mǎn shí​bā suì de rén cái shì hé​gé de xuǎn​mín.

   Only those who have reached eighteen years of age are qualified voters.

   Source: Dong Chinese Dictionary

2. 只有年满十八岁的公民才拥有投票权。

   zhǐ​yǒu nián​mǎn shí​bā suì de gōng​mín cái yōng​yǒu tóu​piào​quán.

   Only citizens who have reached the age of eighteen have the right to vote.

   Source: Dong Chinese Dictionary

3. 只有年满十八岁的公民才有资格参与投票。

   zhǐ​yǒu nián​mǎn shí​bā suì de gōng​mín cái yǒu​zī​gé cān​yù tóu​piào.

   Only citizens who are at least eighteen years old are eligible to vote.

   Source: Dong Chinese Dictionary