As someone who turned 39 this week, I find Sonnet #2 most apposite:
When forty winters shall besiege thy brow And dig deep trenches in thy beauty's field, Thy youth's proud livery, so gaz'd on now, Will be a tatter'd weed, of small worth held: Then being ask'd, where all thy beauty lies, Where all the treasure of thy lusty days, To say, within thine own deep-sunken eyes, Were an all-eating shame and thriftless praise. How much more praise deserv'd thy beauty's use, If thou couldst answer 'This fair child of mine Shall sum my count, and make my old excuse,' Proving his beauty by succession thine! This were to be new made when thou art old, And see thy blood warm when thou feel'st it cold.