One: The Catalyst

Wilbur has always believed the catalyst of being an eldest child is when you turn fifteen.

Tommy is fifteen, and running away from home

Tubbo is (probably) fifteen, and on the run from the government

Ranboo is fifteen, and doesn't recall anything prior to living in the apartment

Wilbur is twenty-two, Techno is nineteen, and Phil is turning forty in September.
And they are chasing a supernova.

TLDR:
Tommy runs away from home to live with his best friends, and his family says: wtf, no small child you cannot do that. And Tommy decides to flip them off in retaliation.

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I posted this on Ao3, and figured it probably was good to post it here too, though idk if anyone reads DSMP fanfics anymore lmao