When you have a character in baby stage it has every challenge in the world because it can do nothing for itself and depends on your for everything, just like a real life baby of course. The challenges really arise when you have a toddler, because just like real life toddlers, they get into everything and still require a lot of attention.
When the character is in the life stage of a child they are still dependant – for instance, they are not able to cook for obvious reasons. Then, when a character is in the stage of a teen they must keep their grades up, get an after school job, remain in a good mood, and eventually aspire to a young adult by going to college. As an adult you can do everything but above all you should care about remaining happy, keeping your aspiration score as high as possible and staying active in order to keep your character alive for as long as possible.