It is clear that:
1- If one, after reaching puberty and the age of religious obligation (which is fifteen in boys) commits this ugly act and is truly penitent and repents to Allah, his repentance will be accepted inshaAllah and will not face punishment in the hereafter.
2- Before reaching puberty, one has no religious obligation.
Nevertheless, it must be kept in mind that the marriage of individuals like this has nothing to do with them coming of age or not.
In the risalahs of the maraje’ it is stated that:
If a man commits liwat with another man, even if it is only limited to the partial insertion of the hashafah  , the mother of the person penetrated and grandmother and so on upwards, and the daughter and granddaughter and so on downwards, and his sister(s) will all become eternally unlawful to him (meaning that he can never get married to any of them) even if the two committing the act aren't balegh (pubescent)  , but mother, sister and daughter of the one penetrating don’t become haram to the one being penetrated.  Of course, if one isn't sure if any penetration actually happened or not or only thinks it might have happened (without being certain), this ruling will not apply.
All of what was mentioned is when liwat takes place before one gets married to the mother, sister or daughter of the partner  , but if a person is married to the mother, sister or daughter of a man, and commits such a sin with him after marriage, it will not invalidate the marriage according to the fatwa of the maraje’  . 
In order to clarify further, an explanation seems necessary: Although, Allah, who has created human beings and knows about all aspects of their lives, has considered it possible for one to be purified from all sins (even if they are major sins) and has called all people to his forgiveness and has promised to forgive them to the extent that he has considered giving up hope in his mercy and forgiveness as the greatest sin, and also, although Allah's prophets were sent to guide people to his unlimited mercy and forgiveness to the extent that the holy prophet of Islam was known as the prophet of mercy and was given the title ‘Prophet of mercy’  ,  but, nevertheless, one mustn’t overlook the reality that there are some sins that inevitably have consequent rulings that cannot be waived even if one achieves true repentance and is forgiven for the sin; this leaves one with no option other than to put up with those consequent outcomes.
For example, one who commits deliberate murder will be forgiven by Allah (swt) inshaAllah if he truly repents to Him, but in the case of the relatives of the victim don’t forgive him, he has to give in to the retaliation penalty (qisas). The same goes for other sins that entail certain rulings and outcomes, such as usurpation, drinking alcohol, masturbation, trampling other’s rights, etc.
The ugly act of liwat, which is actually transgression to oneself, is forgiven if one truly repents, but the ruling it entails remains with that person forever, bringing about certain restrictions for him that he has to cope with.
Makarem: If the one penetrating wasn’t balegh, then there is no eternal unlawfulness.
Safi and Tabrizi: There is eternal unlawfulness as an obligatory precaution if the penetrator was balegh.
Fazel: If a balegh man penetrates a non-balegh boy, if penetration is for sure to have happened, the mother, sister and daughter of the boy will become eternally unlawful to him. But if the penetrator wasn’t balegh, or there is doubt as to whether he was balegh when penetration took place or not, they won't become unlawful to him, and in the case of both being balegh, as an obligatory precaution, they become eternally unlawful to him.
Sistani: The mother, sister and daughter of the penetrated become eternally unlawful to the penetrator in the case of both of them being balegh, even if the glans has only been inserted partially, and the same goes as a necessary precaution if the penetrator is a man or if he isn't balegh, but if one doubts whether penetration actually took place or not, or thinks it might have but isn't sure, then they don’t become unlawful tom him. Also, the mother, sister and daughter of the penetrator don’t become unlawful to the one being penetrated.
 Tahrir al-Wasilah, vol. 2, issue 24, pg. 282.
 Tawdih al-Masa’el, vol. 2, pg. 473, issue 2405.
 Tawdih al-Masa’el, vol. 2, pg. 473, issue 1406.
 Adopted from Question 1246 (website: 1234) (Marriage with the sister of one the one doing liwat).
 Amuzesh Aqa’ed, Misbah Yazdi, pp. 481-482.