The following interpretation is based upon a printing in which 'thy' in line seven is printed 'their.'
The author is the 'decrepit father;' the 'active child' doing 'deeds of youth,' the Shakespeare plays. The author, though personally 'made lame by Fortune's dearest spite,' that is, unable to express himself as himself, yet is consoled by loving, for their own sake, the universal truths in his works. For though one may be raised to high station by being beautiful, or born to it, or wealthy, or intelligent, none of these conditions are guarantees of such place, and so, by loving truth despite any accidents of fortune, the author is freed of fortune's hold. The 'shadow' of the plays is paradoxically the most substantial source of nourishment for the plant of the author. Because he's 'engrafted' to the 'store' of the works, their glory's his, and their greatness grows his happiness.
The author is the 'decrepit father;' the 'active child' doing 'deeds of youth,' the Shakespeare plays. The author, though personally 'made lame by Fortune's dearest spite,' that is, unable to express himself as himself, yet is consoled by loving, for their own sake, the universal truths in his works. For though one may be raised to high station by being beautiful, or born to it, or wealthy, or intelligent, none of these conditions are guarantees of such place, and so, by loving truth despite any accidents of fortune, the author is freed of fortune's hold. The 'shadow' of the plays is paradoxically the most substantial source of nourishment for the plant of the author. Because he's 'engrafted' to the 'store' of the works, their glory's his, and their greatness grows his happiness.