To me, fair friend, you never can be old,
The youth does not seem to have grown older at all in the three years that the poet has known him. Still, age comes on imperceptibly. If so, future ages will have to know that beauty died before future ages were born.
This sonnet deals with the destructive force of time as we grow older. Shakespeare uses his friend as an example. He admires the fact that his friend has kept his youthful appearance over the time that he has known him.
He opens with a sentimental line. He admits that to him his friend will for ever be that young, youthful person. One has to pay careful attention to the way that Shakespeare makes use of words; he says "To me" meaning that such views only apply to him. Also in line one there is an alliteration of the "f" sound "fair friend".
Line three opens up with an exclaimed expression "such seems". The word "such" can suggest that the poet himself is taken aback by this as well. But then the word "seems" suggests that what Shakespeare sees in this friend only appears to be what it is; in reality his friend's appearance has been damaged by time.