The right answer is tienen
Cristóbal y Luis tienen 20 años.
This sentence is given in the simple present tense. In Spanish, this tense is used to talk about habitual actions, routines, things happening now or in the near future, universal truths, facts, hypotheticals, lapses of time, and for ordering in restaurants and stores. There are both regular and irregular indicative forms. The verb tener is irregular, so the conjugation that matches the third-person plural (ellos) is tienen.
