In The Sims 4, Sims can have babies once they become young adults. Young adulthood is the life stage that follows the teenage stage. Once Sims age up from teenagers to young adults, they are eligible to pursue romantic relationships and start a family, including having babies.
It's important to note that in the base game of The Sims 4, Sims cannot become parents as teenagers. They need to age up to young adulthood before they can enter into the family life stage and have children. However, with certain expansion packs or game updates, additional life stages or options for teenage pregnancy may be introduced, enabling teenagers to have babies in specific circumstances.
Overall, in the standard gameplay of The Sims 4 without any additional modifications or content, Sims can have babies once they reach the young adult life stage.