Question
Who are the Sanat Kumaras?
Answer
So are there more than 10 mAnasa putras?
Yes, it seems to be there are more than ten Manasaputra of Lord Brahma. Many Puranas mention it. First Lord Brahma generated some sons but they adopted the path of Nivritti.
Sanandana and the other sons of Brahmā were previously created by him, but they were without desire or passion, inspired with holy wisdom, estranged from the universe, and undesirous of progeny.
Then some other sons had been created.
Brahmā created other mind-born sons, like himself; namely, Bhrigu, Pulastya, Pulaha, Kratu, Aṅgiras, Marīci, Dakṣa, Atri, and Vaśiṣṭha: these are the nine Brahmas (or Brahma ṛṣis) celebrated in the Purāṇas.
Source is Vishnu Purana, Book 1, Chapter 7. There is also an interesting comment given on the same page.
Considerable variety prevails in this list of Prajāpatis, Brahmaputras, Brāhmanas, or Brahmarshis; but the variations are of the nature of additions made to an apparently original enumeration of but seven, whose names generally recur. Thus in the Mahābhārata, Mokṣa Dharma, we have in one place, Marīci, Atri, Aṅgiras, Pulastya, Pulaha, Kratu, and Vaśiṣṭha, ‘the seven highminded sons of the self-born Brahmā.’ In another place of the same, however, we have Dakṣa substituted for Vaśiṣṭha: ‘Brahmā then created mind-begotten sons, of whom Dakṣa was the seventh, with Marīci,’ &c. These seven sons of Brahmā are also identified with the seven Ṛṣis as in the Vāyu; although, with palpable inconsistency, eight are immediately enumerated, or, Bhrigu, Marīci, Atri, Aṅgiras, Pulastya, Pulaha, Kratu, and Vaśiṣṭha. The Uttara Khanda of the Padma P. substitutes Kardama for Vaśiṣṭha. The Bhāgavata includes Dakṣa, enumerating nine. The Matsya agrees with Manu in adding Nārada to the list of our text. The Kūrma P. adds Dharma and Saṅkalpa. The Liṅga, Brahmāṇḍa, and Vāyu P. also add them, and extend the list to Adharma and Ruci. The Hari Vanśa in one place inserts Gautama, and p. 50 in another Manu. Altogether therefore we have seventeen, instead of seven. But the accounts given of the origin of several of these, shew that they were not originally included amongst the Mānasa putras, or sons of Brahmā's mind; for even Dakṣa, who finds a place in all the lists except one of those given in the Mahābhārata, is uniformly said to have sprung from Brahmā's thumb: and the same patriarch, as well as Dharma, is included in some accounts, as in the Bhāgavata and Matsya P., amongst a different series of Brahmā's progeny, or virtues and vices; or, Dakṣa (dexterity), Dharma (virtue), Kāma (desire), Krodha (passion), Lobha (covetousness), Moha (infatuation), Mada (insanity), Pramoda (pleasure), Mrityu (death), and Aṅgaja (lust). These are severally derived from different parts of Brahmā's body: and the Bhagāvata, adding Kardama (soil or sin) to this enumeration, makes him spring from Brahmā's shadow. The simple statement, that the first Prajāpatis sprang from the mind or will of Brahmā, has not contented the depraved taste of the mystics, and in some of the Purāṇas, as the Bhāgavata, Liṅga, and Vāyu, they also are derived from the body of their progenitor; or, Bhrigu from his skin, Marīci from his mind, Atri from his eyes, Aṅgiras from his mouth, Pulastya from his ear, Pulaha from his navel, Kratu from his hand, Vaśiṣṭha from his breath, Dakṣa from his thumb, and Nārada from his hip. They do not exactly agree, however, in the places whence these beings proceed; as for instance, according to the Liṅga, Marīci springs from Brahmā's eyes, not Atri, who there proceeds, instead of Pulastya, from his ears. The Vāyu has also another account of their origin, and states them to have sprung from the fires of a sacrifice offered by Brahmā; an allegorical mode of expressing their probable original, considering them to be in some degree real persons, from the Brahmanical ritual, of which they were the first institutors and observers. The Vāyu P. also states, that besides the seven primitive Ṛṣis, the Prajāpatis are numerous, and specifies Kardama, Kaśyapa, Śeṣa, Vikrānta, Susravas, Bahuputra, Kumāra, Vivaswat, Suchisravas, Prācetasa (Dakṣa), Aṛṣṭanemi, Bahula. These and many others were Prajāpatis. In the beginning of the Mahābhārata (A. P.) we have again a different origin, and first Dakṣa, the son of Pracetas, it is said, had seven sons, after whom the twenty-one Prajāpatis were born, or appeared. According to the commentator, the seven sons of Dakṣa were the allegorical persons Krodha, Tamas, Dama, Vikrita, Aṅgiras, Kardama, and Aswa; and the twenty-one Prajāpatis, the seven usually specified Marīci and the rest, and the fourteen Manus. This looks like a blending of the earlier and later notions.
Though I have not verified all data given above but those seems to be pretty correct.
Are there reference and details of Sanat Kumaras other than the Puranas?
Yes, Sanatkumara are mentioned in other scriptures other than purana like Tezz's commented (in Mahabharata and Chandogya Upanishad), Rickross's answer (Kularnava tantra) and in above source Mahabharata, Book 12, SECTION CCCXLI.
Marichi, Angiras, Atri, Pulastya, Pulaha, Kratu, and Vasishtha,--these seven Rishis have been created by a fiat of the will. These will become the foremost of persons conversant with the Vedas. In fact, they will become the preceptors of the Vedas. They will be wedded to the religion of Pravritti, for they have been intended to devote themselves to the act of procreating offspring. This is the eternal path that I disclose of creatures engaged in acts and observances. The puissant Lord who is charged with the creation of all the worlds is called Aniruddha, Sana, Sanatsujata, Sanaka, Sanandana, Sanatkumara, Kapila, and Sanatana numbering the seventh,--these seven Rishis are known as the spiritual sons of Brahman. Their knowledge comes to them of itself (without being dependant on study or exertion). These seven are wedded to the religion of Nivritti. They are the foremost of all persons conversant with Yoga. They are possessed also of deep knowledge of the Sankhya philosophy. They are preceptors of the scriptures on duty and it is they that introduce the duties of the religion of Nivritti and cause them to flow in the worlds.
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