What's he saying?
"Devouring Time, blunt thou the lion's paws, / And make the earth devour her own sweet brood;"
In time, even lions become weak and lose their powerful claws, and everything dies;
"Pluck the keen teeth from the fierce tiger's jaws, / And burn the long-liv'd phoenix, in her blood;"
Tigers grow old and lose their teeth, and phoenixes burn in their own blood;
"Make glad and sorry seasons as thou fleet'st, / And do whate'er thou wilt, swift-footed Time,"
Time brings the changes of seasons and emotions and does whatever it wants,
"To the wide world and all her fading sweets; / But I forbid thee one most heinous crime:"
Everything in the world is a subject to the passing of time, but Time, there is one thing I won't let you do:
"O! carve not with thy hours my love's fair brow, / Nor draw no lines there with thine antique pen;"
Time, do not make my love's face grow old and wrinkly;
"Him in thy course untainted do allow / For beauty's pattern to succeeding men."
Allow him to remain beautiful, to serve as a paradigm of beauty for all generations.
"Yet, do thy worst old Time: despite thy wrong, / My love shall in my verse ever live young."
But even if you don't listen to me, Time, and you make my love grow old and die, through my poetry he will be immortalized as a young man.
Why is he saying it?
In Sonnet 19, the poet declares his love for the fair lord twice: in line 9, "O! carve not with thy hours my love's fair brow;" and in line 14, "My love shall in my verse ever live young." Though the general belief is that the speaker's attitude toward the fair lord changes in Sonnet 20, the admittance of love for the subject in Sonnet 19 already hints at it. In fact, Sonnets 10, 13, and 15 the speaker has spoken of his love for the fair lord.
The poet addresses Time, making it into a character with whom he pleads. In the first four lines, the poet discusses time's effects on the living things of the world. It "blunts" the paws of the lion, which would have been fearful in youth. Likewise, the "keen teeth" in the tiger's mouth decay with time. Even the phoenix, a mythical bird that lived for hundreds of years before burning itself, then rising with new life from its own ashes - a symbol of immortality - lives out its years in accordance with time.
In lines 9-10, the words "carve" and "draw" suggest that Time is a sculptor or an artist. The speaker pleads with Time to, "carve not with thy hours my love's fair brow, / Nor draw no lines there with thine antique pen." The fair lord's brow would be "carved" with lines and wrinkles as he aged, and this destruction of beauty is regarded by the speaker as the "one most heinous" crime. The pen being "antique" could refer to the age of the pen itself (obviously, it is as old as Time), or of its effects, as it ages people figuratively.
The poet's plea with time is described in lines 11-12: "Him in thy course untainted do allow / For beauty's pattern to succeeding men." The speaker wishes for the fair lord to remain "untainted" by age, though everything else in the world will wither and perish. His reasoning here is that the young man must survive to serve as "beauty's pattern," or an archetype for what true beauty is, to "succeeding men," or future generations.
The theme of the ravages of time is apparent here; now, instead of trying to persuade the fair lord to immortalize himself through procreation, the speaker aims to immortalize the young man himself, through his verse. This solution, however, is not clear until the final couplet of the sonnet, when the speaker gives up trying to convince Time to spare the fair lord, and opts to take action himself: "Yet, do thy worst old Time: despite thy wrong, / My love shall in my verse ever live young."