Vaiṣṇava - On the one hand we see Rādhā and Kṛṣṇa meeting from birth, as Padma Purāṇa says that Rādhikā was born blind and got Her vision only when Kṛṣṇa visited Her and on the other hand in the opening acts of Śrīla Rūpa Gosvāmi's 'Vidagdha-mādhava'-drama, we read about pūrva rāga, how Rādhā-Kṛṣṇa first hear of Each other when they are already in adolescence.
Advaitadās - Yes, Vṛṣabhānu Mahārāja and Nanda Mahārāja are neighbors, presiding over Barsānā and Nandagrām respectively, so it is inconceivable that Rādhā and Kṛṣṇa did not know each other before adolescence. There are several answers to this question.
First of all, Gauḍīya Vaiṣṇavism focuses on nitya līlā, the eternal pastimes, not naimittika līlā, occasional (but in a sense also eternal) pastimes.
Secondly, we must remember that līlā does not take place in linear time as we know it in the material world. Jīva Goswāmī explained that in his commentary on Bhagavata 2.9.10 — kāla-vikrameṇa hi prakṛti-kṣobhāt sattvādayaḥ pṛthak kriyante | tasmāt yatrāsau ṣaḍ-bhāva-vikāra-hetuḥ kāla-vikrama eva na pravartate tatra teṣām abhāvaḥ sutarām eveti bhāvaḥ - “The power of time in Vaikuṇṭha works separately of (beyond) time which is prompted by material modes like sattva. Hence there are no six transformations caused by the power of material time there (birth, growth, full grown state, decay and death).”
What I said just a few seconds ago, that is gone forever. In the meantime I became a few seconds older. That cannot be recovered. You cannot become younger anymore, you cannot turn back the hands of time. Linear time is destructive. The pastimes of the Lord take place outside of the linear time frame. Kṛṣṇa is often addressed in the Bhāgavata with names denoting pastimes He had not performed yet at 'that' time. Durvāsā Muni blessed Śrī Rādhikā that whatever She would cook would give strength and health to Kṛṣṇa. But the līlā of Her cooking for Kṛṣṇa, which happened 'later', as a result of it is also eternal. So how does it work? There is no 'past' before it. How is it possible?
So it is the same with pūrva rāga - Rādhā and Kṛṣṇa always meet again for the first time. This is the point of the whole concept of anurāga. When Rādhikā sees Kṛṣṇa on top of Mount Govardhana She says to Her sakhīs in the Dāna-keli-kaumudī (28)—
prapannaḥ panthānaṁ harir asakṛd asman-nayanayor
apūrvo’yaṁ pūrvaṁ kvacid api na dṛṣṭo madhurimā
pratīke’py ekasya sphurati muhur aṅgasya sakhi yā
śriyas tasyāḥ pātuṁ lavam api samarthā na dṛg iyam
"Kṛṣṇa has crossed the path of My eyes so many times before but I have never before seen such sweetness as today. My eyes cannot drink even a drop of the beauty in one portion of one limb."
So the time-frame of Rādhā-Kṛṣṇa's 'first meeting' does not apply here, and there is also the divine deluding power of Yogamāyā. Even if They had known Each other from birth, they would still be in an illusion of a 'first meeting' during Their adolescence.
We contemplate this aṣṭakāla līlā now for 36 years and we could think 'Oh I know that stuff now, it is old.' But it isn't. Read it and contemplate it again and it is exactly like the first time, but even more attractive than before. vibhur api kalayann sadābhi vṛddhim (Dāna Keli Kaumudi 2) "Though this nectar is all pervading it always increases again still." If we experience ever freshness, then what to speak of Rādhā-Kṛṣṇa, Who are at the pinnacle of anurāga? Every word, every movement there is eternal. If Guruji says you are 12 years 3 months and 15 days old, if you wake up the next morning, you will not be 12.3.16 days old but again 12.3.15 days. It is an eternal loop.
Another thing we must consider is that Kṛṣṇa can perform many different līlās at the same time, as Nārada Muni discovered in the Bhāgavata, when he visited Kṛṣṇa in Dvārakā [Canto 10, chapter 69], and that dramas like Vidagdha Mādhava are templates, blueprints, school-examples to introduce new sādhakas to the different situations and bhāvas in Rādhā-Kṛṣṇa-līlā, rather than a mere continual story.