I jump through some hoops when introducing a new sim to my game. I am in the middle of doing this now for my spellcaster sim having just got RoM.
I start by creating the YA version of the primary sim (or sims if I want siblings) to get the appearance I want for what will be the bulk of my playtime. I then age the sim down to a teen (or a child in some cases) and set traits/clothes etc. I then use genetics to create the parents (sometimes even grandparents).
Where I go from here can differ depending on what I am wanting to do. For my first look at Eco Lifestyle, I moved the teen into the lot on their lonesome and put the parents into Not In World and then commenced playing the teen.
Doing this just gives me a bit of back story should I want it.
