Although the specific meanings of life are specific to specific persons (each in their specific situation) - there is a metaphysical level of understanding of why there is illness, and why there is ageing - and what their functions are, in a general sense.
At an ultimate level; this reality (incarnate mortal life on earth) illness and ageing are inevitable; because this is a reality dominated by entropy. That is: there is a tendency for creation to return to primal chaos.
In this reality - degenerative change is inevitable; hence illness, disease, ageing (and, sooner or later, death).
So in this sense it can reasonably be stated that there is No meaning to illness, disease, ageing. It is something that Just Happens; as a property of the world we live in...
Thus far considered; entropic change is both inevitable and anti-creation, therefore A Bad Thing.
But in fact we are looking upon entropy from the far side of creation. We live in a created reality - our starting point is of creation in-being: creation manifested.
This means that chaos from here is not the same a primal chaos. Primal chaos was neutral - it had no meaning; but chaos from the standpoint of creation is the destruction of exactly that creation which enables us to ask the questions.
If primal chaos is amoral - because morality cannot exist until there is creation; then to return to chaos is a moral act - it is indeed the destruction of the possibility of morality, and therefore anti-moral.
So - here we have a conceptualization of illness, ageing and death as evils; as the destruction of God's creation; as the destruction of Good and even the possibility of Good.
Yet there is yet another perspective. This acknowledges that creation is ongoing. That God's creation is actually a create-ing.
So that disease, ageing and death as they actually occur do so through the medium of God's creating.
This means that we never perceive 'raw entropy' but only the combined effect of entropy with divine creation. Our world is a mixture of the spontaneous consequences of dis-integration and the continual synthesis of creation.
In yet other words; God's creation in this world operates on the raw material of entropy; so that everything we actually perceive is compounded of both...
And therefore partakes of divine purposes and meanings.
What the whole things adds-up-to is: there is always meaning and purpose in the phenomena of degeneration, illness, disease and ageing.
But what that exact meaning and purpose are, is contextual on the individual life in which they occur - depending on the particular nature and circumstances of that life.
And therefore the particular meaning and purpose of some particular instance of degeneration, illness, disease or ageing (why do I have chronic and worsening arthritis, why is my knee swollen, why does it hurt?) is knowable only to someone who has sufficient particular knowledge...
And this particular knowledge must include not only physical but also spiritual aspects; seen in an ultimate context that encompasses resurrected eternal life in Heaven.
So, the meaning of a specific health/age-adversity in a specific person is typically knowable only by that person - and even then, only when that person is able to relate it to the needs of this mortal life in relation to immortal life.
This can crudely be summarized in the question: "What is this trying to teach me?" or "What should I be learning from this?"
And it should not be expected that the real answer be one that we can express to other people, or even one that we can express in language - the answer might well be a word-less and direct apprehension of some-thing.
What is vital is that we learn it - not that we can tell others, or even ourselves, precisely what it is we have learned. That is a quite different, and secondary, matter.