The only reference I can think of is the following verse of The Shatapatha Brahmana.:
tānyetānyaṣṭāvagnirūpāṇi | kumāro navamaḥ saivāgnistrivṛttā ||
18. These then are the eight forms of Agni. Kumara (the boy) is the ninth: that is Agni’s threefold state.
It can also be symbolism to Rudra Kumara but Rudra in vedas means Agni only. Still it's damn close to the Puranic Version of Lord Kartikeya.
Sri Shiva Purana 7.2:2:31.:
70-74. Ṣaṇmukha is born of Śiva. He holds Śakti (Vel) and thunderbolt. He is the son of fire. He is also the son of Gangā, Gaṇāmbā and Kṛttikās. He is surrounded by Viśākha, Śākha and Naigameya. He is the conqueror of Indra and the demon Tāraka. He is the Generalissimo of Indra’s armies. He pierced the important mountains Meru and others with his splendour. He resembles molten gold. He has eyes resembling petals of lotuses. He is Kumāra and the model for all tender beings. He is the favourite of Śiva and fondly attached to Śivā. He is a perpetual worshipper of Śiva’s feet. May he grant my desire at the behest of the two.
I hope this clarifies your queries. Prd..