Birth, Growth, and DeathThe major addition to The Sims 2 is the inclusion of a complete life cycle for your Sims. There are six ages that your Sims will progress through now, each with its own unique demands, tasks, and social interactions.
When your Sim is almost ready to make the transition to a new age, you'll get a small pop-up window letting you know that the change is imminent. This will show up two days before the natural transition day, but when it does appear, you'll be able to make the transition at any time via the "Grow Up" self-interaction, or by blowing out the candles on a birthday cake. In most cases, you'll want to save this transition until the final day, as transitioning early will simply shorten your lifespan. Babies and toddlers, though, being awkward to take care of, should be transitioned as quickly as possible until they become more autonomous children. If you haven't made the transition by 7 p.m. on transition day, it'll occur automatically.
BabyBabies are only babies for three days, but you'll be able to make them transition on the second day of their little existences by having a grownup interact with a birthday cake on their behalf. Babies are difficult to take care of, since they sleep at odd hours and require constant care. You'll have to feed them by getting bottles from the fridge, change their diapers when they mess themselves, play with them to increase their social, and ensure that they get a couple of long naps each day. On the plus side, they're unkillable! They can't die from fire or fly attack or hunger. Superbabies though they may be, if you neglect your baby (reduce its Social or Hunger needs all the way to zero), a social worker will come along and take away all of your kids.
Toddlers exist in this state for up to four days, during which you'll have some important Wants related to their training. If you can teach them potty-training, you won't have to deal with diapers anymore; you can also teach them to walk and talk if you wish for them to be a bit more autonomous. Since they only stay toddlers for four days, however, you'll be hard-pressed to train them in all three of their vital skills without the HomoGenius Smart Milk Aspiration reward item. If you don't manage to get all of their training done, don't worry; this has no impact on their future lives. Children that weren't potty-trained aren't going to wet the bed, for instance. However, completing these Wants will let them make the age transition with a good Aspiration score, which will make for a positive memory.
ChildrenWhen your toddlers become children, they suddenly become much easier to deal with. For one thing, they'll actually sleep in one long block of time, like the older members of your family, instead of two chunks. They'll also start going to school, which will take them out of the house for a goodly amount of time each day. One of the primary tasks of children should be learning how to study; once this is done, they'll be able to do their homework much more quickly, which will let them improve their grades at school and hopefully get into private school that much more easily. You'll also start needing to gain friends for Want-related purposes, so start socializing; if you make friends at this stage of life, you'll have an easier time meeting your friends requirements for your career tracks during the teen and adult years. Children remain children for eight days. Children are also the first group of Sims that can die, albeit only from fire, disease, or drowning.
When a Sim becomes a teenager, his adult life can be truly said to have begun. You won't be able to perform the truly adult interactions, like the Woohoo, but you will be able to start dating and exploring the world of romance. (And they'll almost all want to!) They also start becoming more able to accept responsibility, which will let them train and teach toddlers and children in the household, and will enable them to take on a part-time job, if they wish. Your Sims will remain teens for 15 days, which should give you plenty of time to develop more friendships and get to level three in a career track.
AdultsAchieving adulthood unlocks the most advanced interactions for your character; you're now free to pursue Woohoos, Engagements, Marriage, and so on. In addition, you'll leave school behind and start concentrating on a real career, if you so desire. You remain an adult for 29 days. It's important to note that having a good Aspiration score when you make the transition to elder status will increase your lifetime. If you can make it all the way to platinum when you become an elder, you'll live ten days longer than you normally would!