As per Mahabharata how old was Sri Krishna when he killed Kamsa?

 

While explaining the pros and cons of performing Rajsuya Yagna to Yudhisthira (Sabha Parva), Shri Krishna says that the elders of Bhoja clan requested him in the case of Kaṃsa. And, with the help of Balarama he eliminated Kaṃsa.

kasya cit tv atha kālasya kaṃso nirmathya bāndhavān
bārhadratha sute devyāv upāgacchad vṛthā matiḥ

astiḥ prāptiś ca nāmnā te sahadevānuje 'bale
balena tena sa jñātīn abhibhūya vṛthā matiḥ

śraiṣṭhyaṃ prāptaḥ sa tasyāsīd atīvāpanayo mahān
bhojarājanya vṛddhais tu pīḍyamānair durātmanā

jñātitrāṇam abhīpsadbhir asmat saṃbhāvanā kṛtā
dattvākrūrāya sutanuṃ tām āhuka sutāṃ tadā

saṃkarṣaṇa dvitīyena jñātikāryaṃ mayā kṛtam
hatau kaṃsa sunāmānau mayā rāmeṇa cāpy uta

Some time before, the foolish Kaṃsa, having persecuted the Yadavas, married two of the daughters of Jarasandha. They are called Asti and Prapti and are the sister of Sahadeva. Strengthened by such an alliance, the fool persecuting his relatives gained an ascendency over them all.

But by this conduct he earned great obloquy. The wretch also began to oppress the old kings of the Bhoja tribe, but they, to protect themselves from the persecution of their relative, sought our help.

Having bestowed upon Akrura the handsome daughter of Ahuka, with Sankarshana as my second I did a service to my relatives, for both Kaṃsa and Sunaman were slain by me assisted by Rama.

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If we strictly go by the Mahabharata’s version, Shri Krishna was a fully grown adult at the time eliminating Kaṃsa. However, the exact age of Shri Krishna was not given in Mahabharata.

Please note that after being requested by the elders of Bhoja tribe, Shri Krishna eliminated Kaṃsa.

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If we study Shri Bhagavatam, it says

• Shri Krishna lived in Nanda’s house for eleven years like a covered flame with His elder brother, Baladeva.(3.2.26)
• both Balarama and Shri Krishna were at the age of 11 to 15 years ( adolescent boys - Kiśora), when Akrūra comes to Gokula to bring Shri Krishna to Madhura.

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As Mahabharata preceded all puranas, we can for sure state that Shri Krishna was a fully grown adult when he killed Kaṃsa.