kleśa-vyayāya kalayā sita-kṛṣṇa-keśaḥ
jātaḥ kariṣyati janānupalakṣya-mārgaḥ
This verse is especially describing the appearance of Lord Kṛṣṇa and His immediate expansion, Lord Baladeva. Both Lord Kṛṣṇa and Lord Baladeva are one Supreme Personality of Godhead. The Lord is omnipotent, and He expands Himself in innumerable forms and energies, and the whole unit is known as the one Supreme Brahman. Such extensions of the Lord are divided into two divisions, namely personal and differential. The personal expansions are called the viṣṇu-tattvas, and the differential expansions are called the jīva-tattvas. And in such expansional activity, Lord Baladeva is the first personal expansion of Kṛṣṇa, the Supreme Personality of Godhead.
In the Viṣṇu Purāṇa, as well as in the Mahābhārata, both Kṛṣṇa and Baladeva are mentioned as having beautiful black hair, even in Their advanced age. The Lord is called anupalakṣya-mārgaḥ or, in still more technical Vedic terms, avāṅ-manasā gocaraḥ: one who is never to be seen or realized by the limited sense perception of the people in general. In the Bhagavad-gītā (7.25) it is said by the Lord, nāhaṁ prakāśaḥ sarvasya yoga-māyā-samāvṛtaḥ. In other words, He reserves the right of not being exposed to anyone and everyone. Only the bona fide devotees can know Him by His specific symptoms, and out of many, many such symptoms, one symptom is mentioned here in this verse, that the Lord is sita-kṛṣṇa-keśaḥ, or one who is observed always with beautiful black hair. Both Lord Kṛṣṇa and Lord Baladeva have such hair on Their heads, and thus even in advanced age They appeared like young boys sixteen years old. That is the particular symptom of the Personality of Godhead. In the Brahma-saṁhitā it is stated that although He is the oldest personality among all living entities, He always looks like a new, youthful boy. That is the characteristic of a spiritual body. The material body is symptomized by birth, death, old age and diseases, but the spiritual body is conspicuous by the absence of those symptoms. Living entities who reside in the Vaikuṇṭhalokas in eternal life and bliss have the same type of spiritual body, without being affected by any signs of old age. It is described in the Bhāgavatam (Canto Six) that the party of Viṣṇudūtas who came to deliver Ajāmila from the clutches of the party of Yamarāja appeared like youthful boys, corroborating the description in this verse. It is ascertained thus that the spiritual bodies in the Vaikuṇṭhalokas, either of the Lord or of the other inhabitants, are completely distinct from the material bodies of this world. Therefore, when the Lord descends from that world to this world, He descends in His spiritual body of ātma-māyā, or internal potency, without any touch of the bahiraṅgā-māyā, or external, material energy. The allegation that the impersonal Brahman appears in this material world by accepting a material body is quite absurd. Therefore the Lord, when He comes here, has not a material body, but a spiritual body. The impersonal brahmajyoti is only the glaring effulgence of the body of the Lord, and there is no difference in quality between the body of the Lord and the impersonal ray of the Lord, called brahmajyoti.
Now the question is why the Lord, who is omnipotent, comes here to diminish the burden created upon the world by the unscrupulous kingly order. Certainly the Lord does not need to come here personally for such purposes, but He actually descends to exhibit His transcendental activities in order to encourage His pure devotees, who want to enjoy life by chanting the glories of the Lord. In the Bhagavad-gītā (9.13-14) it is stated that the mahātmās, great devotees of the Lord, take pleasure in chanting of the activities of the Lord. All Vedic literatures are meant for turning one’s attention towards the Lord and His transcendental activities. Thus the activities of the Lord, in His dealings with worldly people, create a subject matter for discussion by His pure devotees.