What do the last two lines in the poem "The Anniversary" mean?

In the first four lines of the first stanza the writer deals with the present and then goes on to say how he and his partner have achieved another year of their reign. The poem continues in the same manner but the last two lines are not about their position as royalty but deals with old age, which, if they both live, they must cope with all the problems of old age, which is the second period of their reign - on this earth. When we are young we cannot begin to contemplate what old age is really like.