I've been told that God Krishna married Rukmini when she was 8 and consummated at same age as per in Skanda Purana. Can I get the right interpretation for this?
Not a chance. The chapter from bhagwat puran gives an answer to this:—
BHAGWATA PURANA. CANTO 10.52.38
A fter the Supreme One who for the sake of His pastimes assumes His bodies, thus had asked these questions, the brahmin related everything to Him. (37) 'S'rî Rukminî told me: 'Oh Most Beautiful One of all the Worlds, I heard about Your qualities. For all who listen and whom You have entered through the openings of their ears, You thus remove the distress of their bodies. To those who have eyes, the sight of Your beauty constitutes the complete fulfillment of their life's purpose. Therefore I have without any shame devoted my mind to You Acyuta! (38) Who oh Mukunda, compares to Your greatness, Your lineage, character, beauty, knowledge, youth, property and influence? Which sober and marriageable girl of a noble birth would, coming of age, not choose You for her husband, oh lion among men, oh You who fills the mind of every member of society with joy? (39) I thus have chosen Your good Self, oh dear Lord, for my husband. I offer myself hereby to You as Your wife oh Omnipotent One. Please accept me! May the king of Cedi [S'is'upâla], who like a jackal wants to steal away the portion belonging to the king of the animals, never touch what is allotted to the [real] hero. (40) When I sufficiently have worshiped the Supreme Personality of Godhead, the Lord, by the performance of pious works, with sacrifices, charity, observances and vows, by honoring the gods, the gurus and the brahmins and with other activities, may [Krishna] the elder brother of Gada [ 9.24: 46] then [please] come and take my hand and not the son of Damaghosa or others like him? (41) Come tomorrow when the marriage takes place, unseen to Vidarbha, oh Invincible One. Fight surrounded by Your officers then to crush the armed resistance of the kings of Caidya and Magadha and next, as the reward for Your valor, marry me in the râkshasa style [by taking me with You]. (42) You may wonder how, with me moving within my quarters, You can carry me away without killing my relatives. Let me tell You how: the day before there is a large ceremonial procession outside [the palace] for the presiding deity of the family. In that ceremony the new bride approaches the goddess Girijâ [ Ambikâ in her temple]. (43) Great souls like [S'iva] the husband of Ûma, in order to overcome their own ignorance, long to bathe in the dust of Your lotus feet. When I, oh Lotus-eyed One, cannot obtain Your mercy, I should, being weakened by vows, give up my life to attain You [only] after hundreds of births.' (44) The brahmin ended with: 'This is the confidential message I bring you, oh Lord of the Yadus, please consider what needs to be done right now in this matter.'
"Which sober and marriageable girl of a noble birth would, coming of age, not choose You for her husband" these verses give explanation and the marriageable girls in india ready to marridge can not anyhow "8".
HARIVAMSA PARVA PROOF
shvo bhAvini vivAhe cha rukmiNI niryayau bahiH | chaturyujA rathenaindre devatAyatane shubhe ||2-59-33
Saying that the marriage can be held tomorrow, rukmiNI went out to the auspicious temple, on the day of jyeShTha star (aindre - see nIlakaNTha commentary), in a chariot yoked with four horses.
indrANImarchayiShyantI kR^itakautukama~NgalA | dIpyamAnena vapuShA balena mahatA vR^itA ||2-59-34
She, with a sparkling body, surrounded by a great army, wearing the auspicious head band, went to worship indrANI.
tAM dadarsha tadA kR^iShNo lakShmIM sAkShAdiva sthitAm | rUpeNAgryeNa sampannAM devatAyatanAntike ||2-59-35
kR^iShNa saw her near the temple as though lakShmI herself was stationed. She was endowed with a beautiful form.
vahnireva shikhAM dIptAM mAyAM bhUmigatAmiva | pR^ithivImiva gaMbhIrAmutthitAM pR^ithivItalAt ||2-59-36
She was blazing like a flame of fire, as an illusion on earth, as the beautiful and majestic (goddess) earth arising out of earth.
marIchimiva somasya saumyAM strIvigrahAm bhuvi | shrImivAgryAM vinA padmaM bhaviShyAM shrIsahAyinIm |
She was beautiful as though the gentle rays of moon has taken the form of woman on earth. She was like lakShmI leaving the lotus. She was to assist (kR^iShNa) as shrI of future.
kR^iShNena manasA dR^iShTAM durnirIkShyAM surairapi ||2-59-37 shyAmAvadAtA sA hyAsItpR^ithuchArvAyatekShaNA |
She who was difficult to be seen even by deva-s, was seen by kR^iShNa with his mind. She was of sixteen years old. She was of white complexion. Her eyes were long and beautiful.
It seems the translation is very correct, no one has given any good refutation of Skanda Purana's translation. I am no expert, bit have been able to understand what some of the original Sanskrit words meant.
The Sanskrit words for the statement that she was eight years old are like this, Englishmeanings in brackets are mine:
Sa(she)kala(time)prayaya(course)aad(?)ashtavarsha (eight years).
Sanskrit source is here, check page number 182 (the Devanagari is labelled 902 in Nepali variant): http://vedicreserve.mum.edu/puranas/skanda_purana/skanda_purana_05avanti_03reva.pdf
Please note that the previous statement was about her birth and her being named Rukmini, so even if its referring to passage of eight years, the translation is basically correct: Yat Survantilkojanmanasahbharat. Tat sa Rukmininambrahmine kirttatada.
The English translation seems correct or close, and the Part 15 of the translation by Sanskrot scholars G.P. Bhatt, Jagdish Lal Shastri, Ganesh Vasudeo Tagare too mentions it as such.
The translation of Harivamsa used here claiming Rukmini was 16 during marriage is incorrect.
There is no Sanksrit word for sixteen mentioned in the transliteration: "shyAmAvadAtA sA hyAsItpR^ithuchArvAyatekShaNA". See here for comparison of Sanksrit words for sixteen:
The Bhagwata Purana is based on mere belief that a mature age was the age of marriage for Hindu girls.
That is false as Manusmriti says even little girls can be married. The Puranas cited here were all written after Manusmriti.
Of course it is not impossible that there may be various versions about her age.