Certainly one key theme of the Sonnets of Shakespeare is the way that so many of them focus as a pageant of the subject's youthful beauty and time's corrosive effects on this beauty. We can obviously infer from this that the speaker of the sonnets is older than their object. In fact, the majority of the poems are written by a male speaker in praise of a youthful male, so most of the sonnets are not actually based around a marriage where the husband is older than the wife. Consider Sonnet 18 and how it presents the theme of beauty and how it will fade. The speaker starts off by saying that in so many different ways, the young man who is the object of this poem is more beautiful than a summer's day, but then it goes on to concede that the youth's beauty will fade just as surely as the beauty of the summer's day. However, his beauty can be made immortal and protected from the ravages of time through the enshrining of his beauty in this verse:
But thy eternal summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou growest...
The speaker of the sonnets, in this sonnet and in other sonnets, thus is able to immortalise the beauty and his love for the youth through the act of writing these poems. The fact that we continue to read and study them today shows how successful he was. Thus the Sonnets definitely deal in a relationship where the speaker is much older than the object of his affection, but the main theme of the Sonnets is the ephemeral nature of beauty and how verse can be used to perpetuate that beauty and protect it from time and death.