Is it easier to obtain an annulment if one has been married for a short period of time? When a couple has been married for many years and raised a family, how can the Church say the marriage did not exist?
Every case is unique and different. Marriage, whether of a short or long duration, is presumed valid until and unless the contrary can be proved with moral certitude. The proofs are determined by a Tribunal process. It does not matter what the length of the marriage was. Morally certain proofs must be established.
A first reaction to a short marriage is that something may have been radically wrong for it to collapse so quickly. But this is only a presumption and it can only be over-ridden by morally certain proofs to the contrary. Certainly, a marriage of long duration (whether or not children were born of the union) can be more difficult to address. Why did the union perdure for so long? The procreation, upbringing and education of a family all suggest a loving, viable and committed relationship between the parents, in particular.
The following analogy might be helpful. Remember it is only an analogy. When the Titanic set sail from England in April of 1912 for its voyage across the Atlantic it is probable it would have safely reached New York had it not collided with the iceberg. In other words it was a worthy seagoing vessel capable of all of the normal encounters with which could be anticipated for any ship on the high seas. The cause of the failure was the collision with the iceberg. Had there been no iceberg, the ship would have had a viable seagoing life. The difficulty in this case was that the presence of the iceberg constituted the setting that caused the ship to sink so quickly. Had the collision with the iceberg been avoided the ship would have reached its destination safely. The matter of its seaworthiness would not have arisen nor would there have been any question about its capacity to fulfill its function as a ship.
On the other hand, if below the waterline, the ship has a serious flaw that will not render it seaworthy, that vessel will be incapable of fulfilling its basic function safely and reliably. In other words it should never be released to sail because it is destined to sink, and will in due course sink, because it is rendered incapable of staying afloat due to the inherent difficulties it will encounter and be unable to cope with in the course of its seagoing life. Ultimately it is going to sink because the basic flaw under the water line has rendered it incapable of staying afloat. Defect of consent can be likened to that basic irreparable flaw under the waterline.
In brief, if the evidence fails to show such a defect (i.e., if the evidence shows that the ship sank because of the iceberg), then the validity of the marriage must be upheld. The icebergs of life do not constitute grounds for a declaration of nullity by the Church.
In a word, no matter what the length of the failed marriage, the Tribunal process must be able to unearth with moral certitude a basic (negative) condition which rendered invalid the marriage in question at the time of the exchange of marital consent between the bride and groom.