I read a list of them in the Catholic Encyclopedia and none of them mentioned anything about immaturity, nor no sufficient reflexion/consideration and the like.

The Novus Ordo is notorious for annulling consummated and valid marriages for reasons such as the above. They say "they didn't realize/know what they were getting into", "they didn't think it through" etc.

What usually happens now is that people get married in a rush or yes maybe when they shouldn't and without thinking it through etc. so then after a little while "problems" start surfacing and they realize they don't get along with each other etc. so they end up wanting an annulment.

Were such things never any valid reasons for annulling a consummated marriage? If it could be granted that yes they didn't fully realize what they were getting into and didn't think it through and shouldn't have gotten married etc., will all of this nevertheless never be any valid reason to get an annulment?

No one EVER knows what they are getting into when they get married!  Problems always arise after a few weeks, or months, or years.  The problem is that we now think marriage should be all sweet and easy, when in fact it can be bitter and disappointing.

I think that is why God "invented" children - to keep a couple together once they realized the truth about how difficult it is to forge a future together.  

That worked quite well until the invention of the Nanny State,  the Daddy State, and the Husband State which promised to support fatherless children and husband-less women womb to tomb.

Marsha

From the Code of Canon Law;

DIRIMENT IMPEDIMENTS IN GENERAL

Can.  1073 A diriment impediment renders a person unqualified to contract marriage validly.

Can.  1074 An impediment which can be proven in the external forum is considered to be public; otherwise it is occult.

Can.  1075 §1. It is only for the supreme authority of the Church to declare authentically when divine law prohibits or nullifies marriage.

§2. Only the supreme authority has the right to establish other impediments for the baptized.

Can.  1076 A custom which introduces a new impediment or is contrary to existing impediments is reprobated.

Can.  1077 §1. In a special case, the local ordinary can prohibit marriage for his own subjects residing anywhere and for all actually present in his own territory but only for a time, for a grave cause, and for as long as the cause continues.

§2. Only the supreme authority of the Church can add a nullifying clause to a prohibition.

Can.  1078 §1. The local ordinary can dispense his own subjects residing anywhere and all actually present in his own territory from all impediments of ecclesiastical law except those whose dispensation is reserved to the Apostolic See.

§2. Impediments whose dispensation is reserved to the Apostolic See are:

1/ the impediment arising from sacred orders or from a public perpetual vow of chastity in a religious institute of pontifical right;

2/ the impediment of crime mentioned in ⇒ can. 1090.

§3. A dispensation is never given from the impediment of consanguinity in the direct line or in the second degree of the collateral line.

Can.  1079 §1. In urgent danger of death, the local ordinary can dispense his own subjects residing anywhere and all actually present in his territory both from the form to be observed in the celebration of marriage and from each and every impediment of ecclesiastical law, whether public or occult, except the impediment arising from the sacred order of presbyterate.

§2. In the same circuмstances mentioned in §1, but only for cases in which the local ordinary cannot be reached, the pastor, the properly delegated sacred minister, and the priest or deacon who assists at marriage according to the norm of ⇒ can. 1116, §2 possess the same power of dispensing.

§3. In danger of death a confessor possesses the power of dispensing from occult impediments for the internal forum, whether within or outside the act of sacramental confession.

§4. In the case mentioned in §2, the local ordinary is not considered accessible if he can be reached only through telegraph or telephone.

Can.  1080 §1. Whenever an impediment is discovered after everything has already been prepared for the wedding, and the marriage cannot be delayed without probable danger of grave harm until a dispensation is obtained from the competent authority, the local ordinary and, provided that the case is occult, all those mentioned in ⇒ can. 1079, §§2-3 when the conditions prescribed therein have been observed possess the power of dispensing from all impediments except those mentioned in ⇒ can. 1078, §2, n. 1.

§2. This power is valid even to convalidate a marriage if there is the same danger in delay and there is insufficient time to make recourse to the Apostolic See or to the local ordinary concerning impediments from which he is able to dispense.

Can.  1081 The pastor or the priest or deacon mentioned in ⇒ can. 1079, §2 is to notify the local ordinary immediately about a dispensation granted for the external forum; it is also to be noted in the marriage register.

Can.  1082 Unless a rescript of the Penitentiary provides otherwise, a dispensation from an occult impediment granted in the non-sacramental internal forum is to be noted in a book which must be kept in the secret archive of the curia; no other dispensation for the external forum is necessary if afterwards the occult impediment becomes public.


http://www.vatican.va/archive/ENG1104/__P3X.HTM

Yes poche, and as you can see nowhere is there any mention of alleged immaturity or the like.