Teens could run away autonomously if they had low relationship with other members of the household. It was not a feature that you could tell your teens to do.
It's part of the game! They have to have a negative relationship with (I think) at least one member of the household. You'll get a notification when they run away.
They could run away from home without mods! Look at this video I just posted on my youtube-channel, where Lilith runs away from the Pleasant household after falling max out with both of her parents. I never play with mods :smiley:
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They may find them and they may not.
If they don't find them, the teen will still age and will return to the house on the day they age up.
https://www.youtube.com/watch?v=76VK3x3yLdI