Lady Capulet utters these words and is often characterized as one of the more vain players on this stage.
What she means with these words as she casts first glance on her dead daughter's corpse is that to see a child of hers already laid to rest in a tomb makes her feel old. Not only that, but also to have lost a child will continue to age her with grey hairs and wrinkles as stress and sorrow always do.
This word sepulcher means to bury. I think she is noting that since she is burying her daughter, her time might not be that far off.