The meaning is that the boy is not affected by the passing of time, that is the lovely boy does not change, he keeps the power of not growing older. As you continue to read the sonnet, you will notice the personna in the poem that is the boy' s lover. She says that the passing of time seems to make him more beautiful, since Nature has given him the power to stop ageing. The personna warns him also that the power to stay young may be taken away from him. It is worth noticing that the power of nature and the passing of time are two main themes in this poem, both represented in the underlined phrase, are present throughout this sonnet as well as other Shakesperean sonnets.
