Devotional service to Krishna is the chief function of the living entity. There are different methods for the liberation of the conditioned soul Karma, jnana, yoga and Bhakti but all are dependent on bhakti. — Sri Caitanya Caritamrita, Madhya lila, 22.17.
It is true that by practicing restraint of the senses by the yoga system one can get relief from the disturbances of desire and lust, but this is not sufficient to give satisfaction to the soul, for this satisfaction is derived from devotional service to the Supreme Personality of Godhead. — Srimad Bhagavatam, 01.16.35.
When transcendental devotional service, by which love for Krishna is attained is executed by the senses, it is called Saadhana Bhakti, or the regulative discharge of devotional service. Such devotion eternally exists within the heart of every living entity. The awakening of this eternal devotion is the potentiality of devotional service in practice. — Sri Caitanya Caritamrta, Madhya Lila, 22.105.
Devotional Service is just like a seed sown in the heart of a living entity and if he goes on hearing & chanting, that seed fructifies, just as the seed of a tree fructifies with regular watering. Thus the spirtitual plant of devotional Service grows gradually.
By rendering devotional service unto the Personality of Godhead, Sri Krishna, one immediately acquires causeless knowledge and detachment from the world. — Srimad Bhagavatam, 01.02.07.
Devotional Service should start at early age, and not during old age.