I used to start with a single young adult all the time, but that got repetitive. I can't do challenges, too many arbitrary rules for me to follow. Plus, I love my mods, most of them are just small mods, they just amend or get rid of things I find annoying or repetitive. Faster eating and drinking, faster showers and baths, no school goals, faster homework, for example
So, in my current game, I started with 3 separate households. All in living in different worlds and with a bit of a backstory, my goal was to have them connect to each other and/or the premades by marriage or friendship. I'm currently in generation 2.
Although I don't do challenges, I did like the idea of an orphanage. So in one game I put down a big house (link below) and found/made a childless adult couple to run it. My goal was to have some of the kids get adopted by premades or townies I created and follow their lives, while others would grow up there and go out on their own for me to play.
https://www.ea.com/games/the-sims/the-sims-4/pc/gallery/738C105F2FEE11E480718FF12688DE2F?category=lots&searchtype=ea_origin_id&sortby=downloads&time=all&searchquery=Maxis&max=50&maxis=false