There are 6 life stages in the sims freeplay, to unlock them all you need to complete a certain main quest. Below I have listed which quest needs to be completed and a link to each of my life stages guides so you can find out all about that life stage:

• Babies– complete the Two and a Half Sims Quest to unlock
• Toddlers– complete A Quest for Toddlers to unlock
• Preteens– complete the Preparing for Preteens Quest to unlock
• Teens– complete Teenagers Quest to unlock
• Adults– complete the Adulthood Quest to age your teens to adults
• Seniors– complete the Seniors Quest to unlock