S2E31 Adult Marco 'the exploding plains of Flendar'

Marco Diaz at age 30.

In the Staar Vs. the Forces of Evil episode, Running with Scissors, Marco Diaz, Star's best friend, crush and host brother abuses the dimensional scissors and angers Hekapoo who takes them away.

Marco is determined to return Star's scissors to her and challenges Hekapoo for the right to use them. Hekapoo challenges him to blow out the flame above her head. With some trickery Marco succeeds but it was one of many clones created by Hekapoo who hide throughout her dimension. Marco then spends the next 16 years tracking her clones. By the time he's 30, his survival skills and time in the wilderness tone his body. He finds the real Hekapoo and blows out her flame. Star arrives and after being briefly awwed by the adult Marco's muscles and handsomeness, tells him that while he spent 16 years in Hekapoo's dimension, only 8 minutes passed on Earth.

Marco likes his life but is convinced to return. Back on Earth and its timeline, he reverts back to a 14 year-old.