I usually leave my settings at the default epic, but I'm beginning to change my mind about that. For me, it's not about duration. If you want generations to fly by, pick a shorter span. For me, it's about proportion. If a sim is an infant for five days, five days should count for a year if you want it to resemble real life. Toddler stage should then be 10 days, ages 1-3. Child should be 50 days, ages 3-12. Teen should be 35 days, or a little less if you decide 18 years is a young adult. Exactly where the other stages fall depends on your perception of where those stages begin and end. The game is centered around young adults and, to a lesser extent, teens and adults, to appeal to the casual simmer who doesn't play muti-generational families. There's a limit to "realistic," but I think the age settings as they are don't come very close.