When did animals begin to die? When did animals begin to die?
Did animals begin slowly dying at the time of the fall of Adam and Eve or had it happened earlier at the fall of Satan, which may be what Romans 8:20 is hinting at?

For the creature was made subject to vanity, not willingly, but by
reason of him who hath subjected it (Rom 8:20)

Were animals already dying when God gave humans to have dominion over them in Gen. 1:28?:

And God blessed them, and God said unto them, Be fruitful, and
multiply, and replenish the earth, and subdue it: and have dominion
over the fish of the sea, and over the fowl of the air, and over every
living thing that moveth upon the earth. (Gen. 1:28, KJV)

Or were animals originally created by God as already dying?
What do the Roman Catholic Church, the Eastern Orthodox Church, and the main-stream Protestants -- that is, those Protestants who pray to Jesus Christ in their prayers -- officially teach regarding this matter?
EDIT 4: Following one remark in comments (stating that senescence by definition is not only the process of aging, but also the process of maturing that precedes the process of aging) decided to switch to &quot;dying&quot; again;
EDIT 3: Following a suggestion in the comments, rephrased the question as &quot;When did senescence enter the animal kingdom?&quot;
EDIT 2: Originally I used the term &quot;mortal&quot; in my question, but since the word &quot;mortal&quot; may mean only &quot;being able to be killed&quot; (see sequoia trees example in the EDIT 1 or Adam before his fall), I decided to switch to &quot;dying&quot;.
EDIT 1: By &quot;dying&quot; or &quot;already dying&quot; I mean the very state, in which all animals are now: each species is born, lives a certain number of years, gets old and then dies. This is different from being able to be killed, yet not having the process of dying taking place within (consider sequoia trees - some of them don't die by themselves, but can be killed by, say, fire or being cut)
When did senescence enter the animal kingdom?
To speak openly we may not know the exact moment when they became dying? But we have a few key points that the Scriptures can help in giving some perspectives on the question.
Immediately after Adam and Eve sinned, we are told that they made clothing for themselves.

Then the eyes of both were opened, and they knew that they were naked. And they sewed fig leaves together and made themselves loincloths. Genesis 3:7 ESV

However, after God comes into the garden, He also makes clothing for them.

And the Lord God made for Adam and for his wife garments of skins and clothed them. - Genesis 3:21 ESV

This seems to indicate that animals seemed to die immediately after the fall of Adam. But we do not know definitively in this was actually the case.
When one asks the question whether animals ”become dying at the time of the fall of Adam and Eve or had it happened earlier at the fall of Satan, which may be what Romans 8:20 is hinting at?”

For the creation was subjected to frustration, not by its own choice, but by the will of the one who subjected it, in hope. (Rom 8:20)

The material universe was created by God and when man was created, he was given dominion over God’s creation; but when Adam and Eve sinned, sin and death entered the world.

12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
13 (For until the law sin was in the world: but sin is not imputed when there is no law.
14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.
17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)
18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous. - Romans 5:12-21

St. Paul alludes to this in Romans:

For we know that the whole creation groans together and travails in pain together until now. - Romans 8:22

Ultimately we may never truly know or understand when animals become dying?
Many Catholics, Orthodox and other fellow Christians  may believe in evolution. In this scenario, it would be obvious the animals died before sin entered the world. What  percentage of Christian hold to this theory I am not aware of.
The Catholic Church holds no official position on the theory of creation or evolution, leaving the specifics of either theistic evolution or literal creationism to the individual within certain parameters established by the Church. According to the Catechism of the Catholic Church, any believer may accept either literal or special creation within the period of an actual six-day, twenty-four-hour period, or they may accept the belief that the earth evolved over time under the guidance of God. Catholicism holds that God initiated and continued the process of his evolutionary creation and that all humans, whether specially created or evolved, have and have always had specially created souls for each individual.
To say the least, the fact is that the Church has been very comfortable with evolution as a scientific theory since at least the pontificate of Pius XII and his encyclical Humani Generis (1950). Popes John Paul II and Benedict XVI in particular have made strong statements in favour of evolution and its compatibility with Christian teaching about God and creation.


For these reasons the Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter - for the Catholic faith obliges us to hold that souls are immediately created by God. However, this must be done in such a way that the reasons for both opinions, that is, those favorable and those unfavorable to evolution, be weighed and judged with the necessary seriousness, moderation and measure, and provided that all are prepared to submit to the judgment of the Church, to whom Christ has given the mission of interpreting authentically the Sacred Scriptures and of defending the dogmas of faith. Some however, rashly transgress this liberty of discussion, when they act as if the origin of the human body from pre-existing and living matter were already completely certain and proved by the facts which have been discovered up to now and by reasoning on those facts, and as if there were nothing in the sources of divine revelation which demands the greatest moderation and caution in this question.

When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents. Now it is in no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own.



This evolution theory is to some degree accepted by major Christian churches, including the Catholic Church, Evangelical Lutheran Church in America, Episcopal Church (United States), and some other mainline Protestant denominations; virtually all Jewish denominations; and other religious groups that lack a literalist stance concerning some holy scriptures. Various biblical literalists have accepted or noted openness to this stance, including theologian B.B. Warfield and evangelist Billy Graham.
See: Acceptance of evolution by religious groups.
For further information, one may peruse the following articles at one’s leisure:

Did Animals Die before the Fall?
Death before Sin?
Animal Death Before the Fall: What Does the Bible Say?