Dhruva Mahārāja is described herein as kṣatra-bandhuḥ, which indicates that he was not fully trained as a kṣatriya because he was only five years old; he was not a mature kṣatriya. A kṣatriya or brāhmaṇa has to take training. A boy born in the family of a brāhmaṇa is not immediately a brāhmaṇa; he has to take up the training and the purificatory process.
The great sage Nārada Muni was very proud of having a devotee-disciple like Dhruva Mahārāja. He had many other disciples, but he was very pleased with Dhruva Mahārāja because in one lifetime, by dint of his severe penances and austerities, he had achieved Vaikuṇṭha, which was never achieved by any other king's son or rājarṣi throughout the whole universe. There is the instance of the great King Bharata, who was also a great devotee, but he attained Vaikuṇṭhaloka in three lives. In the first life, although he executed austerities in the forest, he became a victim of too much affection for a small deer, and in his next life he had to take birth as a deer. Although he had a deer's body, he remembered his spiritual position, but he still had to wait until the next life for perfection. In the next life he took birth as Jaḍa Bharata. Of course, in that life he was completely freed from all material entanglement, and he attained perfection and was elevated to Vaikuṇṭhaloka. The lesson from the life of Dhruva Mahārāja is that if one likes, one can attain Vaikuṇṭhaloka in one life, without waiting for many other lives. My Guru Mahārāja, Śrī Śrīmad Bhaktisiddhānta Sarasvatī Gosvāmī Prabhupāda, used to say that every one of his disciples could attain Vaikuṇṭhaloka in this life, without waiting for another life to execute devotional service. One simply has to become as serious and sincere as Dhruva Mahārāja; then it is quite possible to attain Vaikuṇṭhaloka and go back home, back to Godhead, in one life.