This Answer mention Ganesha as the elder brother of Kartikeya. But most of the North Indians believe that Kartikeya is elder. So, is there any scripture which mention Kartikeya as the elder brother of Ganesha?
This excerpt from the Kumara Khanda of the Shiva Purana describes Kartikeya participating in the battle between Shiva and Ganesha that happened right after Ganesha was born:
[Shiva] issued directives to Indra and other gods, to the Ganas led by the six-faced Kumara and to goblins, ghosts and spirits. At the bidding of Shiva they all desired to kill Ganesha. Lifting their weapons in a suitable manner hey came there from all directions.
So in this account, Kartikeya is older than Ganesha.