The inference that can be drawn from these lines of Feste's song in Shakespeare's Twelfth Night is that they are making reference to life’s unpredictability and; therefore, they urge people to enjoy their youth.
This excerpt from Twelfth Night, a comedy written by Shakespeare, is part of what Feste sings after Sir Toby and Sir Andre Agueecheeck ask him to sing a love song. He is telling lovers not to lose time because life is unpredictable; in other words, no one knows what will happen tomorrow ("what's to come is still unsure"). Moreover, he is saying that youth will not last forever. For that reason, he advises lovers to make the most out of the present ("Then come kiss me, sweet and twenty"). Therefore, the idea expressed in this excerpt is closely connected to the theme of carpe diem, which means "seize the day".
