(There are other such old ganesha idols, many rock-cut ones in Tamil Nadu) As wikipedia says: The family includes his brother the war god Kartikeya, who is also called Skanda and Murugan. read more
In the north and according to a few puranas Kartikeya is considered elder while in the South and some Puranas consider Ganesha elder. Which Puranas you prefer determines their seniority. read more
Simply stated, the traditional belief in the area North of the Vindhyas, is that Ganesh is the younger and Kartikeya (Subrahmanyam/Murugan), the elder of the sons of Shiva parivar. The tradition in South India is vice-versa. read more