Always engage in devotional service with determination.
Devotional Service should start at early age, and not during old age.
In the preliminary phase of spiritual life there are diffrent kinds of austerities, penances and similar processes for attaning self-realization. However, even if an executor of these processes is without any material desire, he still cannot achieve devotional service and aspiring by oneself alone to achieve devotional service is also not very hopeful, because Krishna does not agree to award devotional service to merely anyone. Krishna can easily offer a person material happiness or even liberation, but he does not agree very easily to award a person engagement in His devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee.
Speculative knowledge alone, without devotional service, is not able to give liberation. On the other hand, even without knowldege one can obtain liberation if one engages in the Lord's devotional service. — Sri Chaitanya Caritamrita, Madhya Lila, 22.21.