1-2. As fast as thou shalt wane, so fast thou growest / In one of thine, from that which thou departest: —Shakespeare here wrestles the syntax to own will, and I'm not quite sure, but I think the whole thing means, "as fast as you lose yourself (through age), just so fast will a child of yours recreate you as he grows to maturity."

3. youngly: in youth.

4. convertest: change (to old age).

5. Herein: i.e., in producing a child.

7. minded so: of your mind (to remain childless). times should cease: i.e., human history would come to an end (because there would be no more people).

9. store: a stock to draw upon.

10. Harsh: displeasing to the eye. featureless: ill-favored, ugly. rude: crudely formed.

11. Look whom: whomever.  —As far as I can see, the rest of the line is redundant; the poet seems to be saying that those to whom Nature gives the best gifts she gives the most.

12. in bounty: i.e., by being bountiful.

13. carved: fashioned, intended.  seal: stamp used to make impressions.

14. copy: original (as in "printer's copy").