Ignatius Theophorus's answer to this question about the assumption of Mary states:
In addition, there is an ancient document which actually relates the death of Mary and the assumption of her body.
When I heard about the Catholic belief that Mary was sinless and was assumed into heaven, I thought the two beliefs fit together well since death is a consequence of sin. However, if it is taught she did die (or even would have died), how is this reconciled with "the wages of sin is death"?
Orthodoxy has a Tradition of the Dormition of Mary wherein it is belived that she didn't die but merely fell asleep and was assumed into Heaven. Catholic teaching neither affirms nor contradicts the Dormition but we have another tradition which is written in one of the apocryphal gospels ascribed to St. Thomas but written hundreds of years a after his death.
Here is the ancient document alluded to. And the very ancient and venerable and understandable tradition it is in keeping with is that The Blessed Virgin herself didn't wish to in any way not be a perfect disciple and imitator of her Son. So if he chooses to die, she would choose to die as well.
I've got the same questions as to what it exactly means for Mary to have died, but it is a mystery of the Church and one you can contemplate here and ask about in christianity.stackexchange.hvn in the life of the world to come to get a better answer.
Full of grace, preserved from original sin, Mary never experienced its consequences, and at the end of her earthly life she was taken up body and soul into heaven, where we contemplate her as Our Lady of the Angels and Queen of the Universe.
Pope John Paul II - Angelus address on Solemnity of the Assumption (August 15 1998)
In the fullness of time, at the end of her earthly life, he would call her to follow him completely into the reality of resurrection. That is what the Assumption is all about. In Mary assumed into heavenly glory, the little one of Nazareth is now resplendent in the divine light. In her we see our eternal destiny, the destiny of the church.
Most Rev. Peter J. Elliott, KC*HSJ - Retreat Homily - 2009
In some of the writings of the Church Fathers we find Jesus himself described as coming to take his Mother at the time of her death to bring her into heavenly glory. In this way they present the death of Mary as an event of love which conducted her to her divine Son to share his immortal life. At the end of her earthly life, she must have experienced, like Paul and more strongly, the desire to be freed from her body in order to be with Christ for ever (cf. Phil 1:23).
The experience of death personally enriched the Blessed Virgin: by undergoing mankind’s common destiny, she can more effectively exercise her spiritual motherhood towards those approaching the last moment of their life.
Pope John Paul II - General Audience - June 25 1997
The last link addresses very straightforward Our Lady in connection with earthly death and probably succinctly answers the question. Catholic doctrine merely says "At the end of her earthly life" it doesn't say, "shortly after her soul left her body" or "when rigor mortis set in". We wholeheartedly believe that she suffered no decay due to the wages of sin nor would she had she remained on earth after death for 1000 years before being brought into heaven.
It is NOT the official catholic teaching that Mary died. When Pope Pius XII in 1870 defined the Assumption of the Virgin Mary as dogma, he deliberately left open the question of whether Mary died before her Assumption.
... The Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.src
So a Catholic can either hold that Mary died, Chose to die or went into a deep sleep. None of these will contradict any of the Catholic teaching or Scripture.
Yes, Scripture too. Justification for this can be found in Ignatius Theophorus's answer.
OK. Couple of things.
For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord. (RSV)
Clearly "life" and "death" in that verse do not refer to this mortal coil.
Now, I don't remember the source (so this could be completely made up), but I seem to recall hearing some source of at least some reliability (a saint's vision or an ancient legend) which claimed that Mary was given the choice whether to die or not. It then continued to state that Mary chose to die because that would more unite her with Christ.
The key to answering your question is an exact understanding of the statement that Mary was sinless, which covers both original sin and personal sins. The absence of personal sins is clearly not relevant to this subject, because babies die. Thus, we have to focus on original sin, which has several consequences:
Deprivation of sanctifying grace and charity. This is precisely "original sin" itself, and is what Mary never had according to Catholic dogma. However, the reason why she was never deprived of sanctifying grace was because the foreseen merits of Christ were applied to her at the time of her conception. It is as if she was conceived already baptized, not as if she naturally did not need baptism.
Concupiscence. That Mary was additionally free from concupiscence is not part of the dogma but is common Catholic doctrine [1], and quite reasonable if Mary did not commit even one venial sin in her whole life, which is Catholic dogma.
Mortality and passibility. Before original sin, Adam and Eve were not going to die, get sick, age past a certain optimal point (which I reckon at 3x years), or be attacked by predators. Just as the application of Christ's merits at baptism does not include the restoration of these preternatural gifts to babies, the preemptive application of Christ's merits to Mary did not necessarily include that restoration to her [2]. Which is completely different from the case of Jesus, for Whom there was nothing to restore.
[1] John Paul II, General Audience, 12 June 1996.
Pius IX's definition refers only to the freedom from original sin and does not explicitly include the freedom from concupiscence. Nevertheless, Mary's complete preservation from every stain of sin also has as a consequence her freedom from concupiscence, a disordered tendency which, according to the Council of Trent, comes from sin and inclines to sin (DS 1515).
http://www.ewtn.com/library/papaldoc/jp2bvm23.htm
[2] John Paul II, General Audience, 25 June 1997.
Some theologians have in fact maintained that the Blessed Virgin did not die and was immediately raised from earthly life to heavenly glory. However, this opinion was unknown until the 17th century, whereas a common tradition actually exists which sees Mary's death as her entry into heavenly glory. [...] However, the fact that the Church proclaims Mary free from original sin by a unique divine privilege does not lead to the conclusion that she also received physical immortality.
http://www.ewtn.com/library/PAPALDOC/JP2BVM53.HTM
http://www.totus2us.com/vocation/mary-most-holy/jpii-catechesis/the-dormition-of-the-mother-of-god/
