"Sonnet 15" continues the theme of the life cycle from vibrant youth to harrowing old age, the joys of reproduction and the happiness as well as the sadness of passing on seeds to a new generation.
The line "Whereon the stars in secret influence comment;/" appears to represent fate watching over the "men as plants increase, cheered and checked even by the selfsame sky." The new breed of flora, the "men" or people as a whole, can be encouraged, cheered on because they are the new generation to carry out new deeds, new operations, new identities to carry out civilization's whims and wills. It can be said that this new breed could be "checked," held back by shortcomings of what the previous generation suffered, that the new generation would have to carry the sins of the mothers and the fathers on their backs.
The theme of the aging process also shows up in "Sonnet 15." "Where wasteful Time debateth with Decay," could mean that both these factors fight against the new generation, and possibly the poet. Time makes us all age, and we all must face decay not just in the aging process but with whatever misfortunes life throws on the road. The next set of lines offer comfort in the struggle: "And, all in war with Time for love of you,/As he takes from you, I engraft you new." Time may be raging against the flesh and the mind, but there is hopes for further reproduction, for a new generation to pop up, to carry on the usual deeds and possibly do better things than the previous generation did with some "engrafting," new life surging into the veins of people.