How did you decide on age groups?

My husband and I decided betwen 3 to 6 because we can interact with them and litle babies kind of scare me a litle bit since i have 0 experience and older kids I dont think I will be able to manage if they behave bad and I dream of a litle girl no a baby no a teeneger just a litle girl I know the age have their own challenges but also depende on each case I think. If your husband dream of a litle budy and you adopt a litle kid eventually he will have his teen son. :)