sonnetXI

As fast as thou shalt wane, so fast thou grow'st
In one of thine, from that which thou departest;
And that fresh blood which youngly thou bestow'st,
Thou mayst call thine when thou from youth convertest.
Herein lives wisdom, beauty, and increase;
Without this folly, age, and cold decay:
If all were minded so, the times should cease
And threescore year would make the world away.
Let those whom nature hath not made for store,
Harsh, featureless, and rude, barrenly perish:
Look whom she best endowed, she gave the more;
Which bounteous gift thou shouldst in bounty cherish:
   She carved thee for her seal, and meant thereby,
   Thou shouldst print more, not let that copy die.

This takes up the same argument as sonnet I, and indeed it is a sonnet that can hardly stand on its own because 'if all were minded so' implies that we know already how the youth is minded to behave, information which we only derive from what has gone before.

'Youth rapidly wanes, but this decrease may be made up by the children that a young man may beget. Otherwise all are doomed to age and decay. Since you are manifestly so beautiful, let the fate of dying out be left to barren, harsh and sullen souls. You are so well endowed by nature, that it is clear she intended you to be a seal, from the impress of which many copies should be made'.

The 1609 Quarto Version

AS faſt as thou ſhalt wane ſo faſt thou grow'ſt,
In one of thine,from that which thou departeſt,
And that freſh bloud which yongly thou beſtow'ſt,
Thou maiſt call thine,when thou from youth conuerteſt,
Herein liues wiſdome,beauty,and increaſe,
Without this follie,age,and could decay,
If all were minded ſo,the times ſhould ceaſe,
And threeſcore yeare would make the world away:
Let thoſe whom nature hath not made for ſtore,
Harſh,featureleſſe,and rude , barrenly perriſh,
Looke whom ſhe beſt indow'd,ſhe gaue the more;
Which bountious guift thou ſhouldſt in bounty cherriſh,
   She caru'd thee for her ſeale,and ment therby,
   Thou ſhouldſt print more,not let that coppy die.

Commentary

1. As fast as thou shalt wane, so fast thou grow'st

As rapidly as you decline, your descendants would grow accordingly. wane is used of the moon, but also of power, success, and other worldly matters.

2. In one of thine, from that which thou departest;
In one of thine = in a child of yours;
from that which thou departest = as your increasing age takes you away from your presently beautiful self. With the sense also that he is departing (in age) from the child he leaves behind.
3. And that fresh blood which youngly thou bestow'st,
blood here used in the figurative sense of family, life, descendants, seed, semen, (hence bestow'st, to bestow = to give, to grant, to lay out, to expend).
4. Thou mayst call thine when thou from youth convertest.
And these, the children you beget, you may call your own (in the sense also that they carry with them the beauty of your youth), when you yourself are turning away (convertest) and leaving behind your young days. convert = to turn from. From the Latin convertere to make to turn round.
5. Herein lives wisdom, beauty, and increase;

Herein -  in (by) doing this, in taking on the responsibility of breeding, as we have all been urging you to do. increase = abundance, fruition.

6. Without this folly, age, and cold decay:
Without this = Without this policy, intention, desire (for continuing your line) there lives folly etc. without also has the meaning of 'outside', and is contrasted here with herein. Hence 'outside this accepted norm of behaviour lies folly, age, decay'.
7. If all were minded so, the times should cease
If all were minded so = If everyone were of the same mind as you (i.e. not to have children).
the times = historical ages?, time itself ?
should cease = would come to an end.
8. And threescore year would make the world away.

threescore being a typical lifespan, being rounded down from the biblical threescore and ten. year is an old plural form of 'years', still found in phrases such as 'for many a year' which is equivalent to 'for many years'.
would make the world away = would bring the world to an end, with the suggestion of killing it, as in 'to make away' with someone, to do them in.

9. Let those whom nature hath not made for store,
made for store = made for breeding. The best things are preserved, stored, for future use. Store-cattle is a term still used, meaning cattle kept for breeding. It is a policy in husbandry to keep the best lines for breeding the next generation.
10. Harsh, featureless, and rude, barrenly perish:
This describes those who are not to be kept for store. Let them perish without issue. There is a slightly eugenic flavour to this wish, but it is linked to the hyperbole that the youth, above all other things in the world, is worthy to be copied. If he does not allow it, the world would be deprived of his incomparable qualities.
rude = coarse, brutish.
11. Look whom she best endowed, she gave the more;
Look whom = whoever, whatever persons; See Sonn.9, note to line 9.
she = Nature;
best endow'd = gave the best qualities to, gave most abundantly to;
she gave the more = she (Nature) gave even more in addition to what she had already bestowed. The sentence seems to be tautological, and is said to echo Matt.25.29, where, however, the emphasis appears to be that from those who have nothing or very little that little is to be stripped. The emendation of the to thee has been proposed, which gives the meaning that Nature has given even more to the youth than to those to whom she has otherwise generously bestowed her gifts.
12. Which bounteous gift thou shouldst in bounty cherish:
which bounteous gift = Nature's generous bestowal of qualities upon you.
in bounty = with (equal) generosity.
cherish = value, respect.
13. She carved thee for her seal, and meant thereby,
Nature made you as an example to the world (of beauty) as if she were making a seal. The seal here is the instrument from which an impression would be stamped out in wax of whatever was depicted on the seal (as, for example, the royal coat of arms on the soverign's seal).
meant thereby = intended that from it (the seal).
14. Thou shouldst print more, not let that copy die.
It is your obligation to make (print) copies of yourself, not to let the original copy or template perish. Cf.: All's Well, the King speaking of Bertram's father:
........Such a man
Might be a copy to these younger times;
Which, follow'd well, would demonstrate them now
But goers backward.
AWW.I.2.45-8.