SB 10.45.3

nāsmatto yuvayos tāta
 nityotkaṇṭhitayor api
bālya-paugaṇḍa-kaiśorāḥ
 putrābhyām abhavan kvacit
Synonyms: 
na — not; asmattaḥ — because of Us; yuvayoḥ — for you two; tāta — O dear father; nitya — always; utkaṇṭhitayoḥ — who have been in anxiety; api — indeed; bālya — (the pleasures of) the toddler age; paugaṇḍa — boyhood; kaiśoraḥ — and youth; putrābhyām — because of your two sons; abhavan — there were; kvacit — at all.
Translation: 
[Lord Kṛṣṇa said:] Dear Father, because of Us, your two sons, you and mother Devakī always remained in anxiety and could never enjoy Our childhood, boyhood or youth.
Purport: 

Śrīla Viśvanātha Cakravartī discusses this verse as follows: “One may object that at this point Lord Kṛṣṇa had not actually passed the kaiśora stage [age ten to fifteen], since the women of Mathurā had stated, kva cāti-sukumārāṅgau kiśorau nāpta-yauvanau: ‘Kṛṣṇa and Balarāma have very tender limbs, being still at the kiśora stage, not having reached adolescence.’ (Bhāg. 10.44.8) The definition of the different stages of growing up is given as follows:

kaumāraṁ pañcamābdāntaṁ
 paugaṇḍaṁ daśamāvadhi
kaiśoram ā-pañcadaśād
 yauvanaṁ tu tataḥ param

‘The kaumāra stage lasts until the age of five, paugaṇḍa up to age ten and kaiśora to age fifteen. From then on, one is known as yauvana.’ According to this statement, the kaiśora period ends at the age of fifteen. Kṛṣṇa was only eleven years old when He killed Kaṁsa, according to Uddhava’s words: ekādaśa-samās tatra gūḍhārciḥ sa-balo ’vasat. ‘Like a covered flame, Lord Kṛṣṇa remained there incognito with Balarāma for eleven years.’ (Bhāg. 3.2.26) And since Kṛṣṇa and Balarāma never took brahminical initiation in Vraja-bhūmi, it was at the time [of Their going to Mathurā] that Their kaiśora stage began rather than ended.

“This objection to Lord Kṛṣṇa’s statement in the present verse — that His parents could not enjoy His kaiśora stage — is based on ordinary measurement of age. Yet we should consider the following statement:

kālenālpena rājarṣe
 rāmaḥ kṛṣṇaś ca go-vraje
aghṛṣṭa-jānubhiḥ padbhir
 vicakramatur añjasā

‘O King Parīkṣit, within a short time Rāma and Kṛṣṇa began to walk very easily in Gokula on Their legs, by Their own strength, without the need to crawl.’ Sometimes we see that the son of a king, even while in his paugaṇḍa stage of life, undergoes exceptional physical growth and exhibits activities appropriate to a kaiśora. Then what to speak of Lord Kṛṣṇa, whose exceptional growth is established in the Vaiṣṇava-toṣaṇī, Bhakti-rasāmṛta-sindhu, Ānanda-vṛndavana-campū and other works?

“The three years and four months that Lord Kṛṣṇa stayed in Mahāvana were the equivalent of five years for an ordinary child, and thus in that period He completed His kaumāra stage of childhood. The period from then to the age of six years and eight months, during which He lived in Vṛndāvana, constitutes His paugaṇḍa stage. And the period from the age of six years and eight months through His tenth year, during which time He lived in Nandīśvara [Nandagrāma], constitutes His kaiśora stage. Then, at the age of ten years and seven months, on the eleventh lunar day of the dark fortnight of the month of Caitra, He went to Mathurā, and on the fourteenth day thereafter He killed Kaṁsa. Thus He completed His kaiśora period at age ten, and He eternally remains at that age. In other words, we should understand that from this point on the Lord remains forever a kiśora.

Thus Śrīla Viśvanātha Cakravartī analyzes the intricacies of this verse.