To me, fair friend, you never can be old,
For as you were when first your eye I ey'd,
Such seems your beauty still. Three winters cold,
Have from the forests shook three summers' pride,
Three beauteous springs to yellow autumn turned
In process of the seasons have I seen,
Three April pérfumes in three hot Junes burned,
Since first I saw you fresh, which yet are green.
Ah! yet doth beauty like a dial-hand,
Steal from his figure, and no pace perceived;
So your sweet hue, which methinks still doth stand,
The Fair Youth is likened to the seasons. Despite the effect of hot weather on the rest of the world, the boy is “yet … green”. 
 The reference to the colour green is layered, implying freshness and richness, but also innocence.  In  Antony and Cleopatra  the Queen of Egypt refers to her  salad days when I was green in judgment ; that is, when she was immature and lacking sophistication.
Sonnet 104