1. glass: mirror.

3. fresh repair: youthful condition.

4. beguile: cheat. unbless some mother: deprive some woman of the blessing of motherhood.

5. unear'd: untilled, uncultivated.

6. husbandry: cultivation. —There's also an pun on "husband."

7. so fond will: so foolish that he will.

8. Of: because of.  stop posterity: i.e., to prevent yourself from being remembered.

9. glass: mirror.

10. prime: springtime, youth.

11. windows of thine age : i.e., children, through whom he will be able to see himself as he is now, in youth.

13. live rememb'red not to be: i.e., live in such a way that you will be remembered only as one who barely existed.