According to the Paddhati of Dhyanacandra Gosvami (he refers to Brhad-gautamiya-tantra), the eternal ages (in nitya-lila) of Sri Radha and Sri Krishna are as follows:
Sri Radha: 14 years, 2 months, 15 days
Sri Krishna: 15 years, 9 months, 7 days
In other words, Sri Krishna is older than Radha, not the other way around. It would create an awkward situation of rasa if it was so.
In his commentary on Bhakti-rasamrta-sindhu, Sri Visvanatha Cakravartipada gives the following course of Krishna’s age and his pastimes:
Gokula Mahavana: Birth – 3 years 4 months
Vrindavan, Raman Reti: 3 years 4 months – 6 years 8 months
Nandagram: 6 years 8 months – 10 years 7 months
At the age of 10 years, 7 months, on the dark fortnight of the month of Caitra, Sri Krishna left to Mathura.
Sri Visvanatha Cakravartipada further explains that He stayed in Mathura until the age of 28 years, four months, and then moved over to Dvaraka.
In the Vaisnava Tosani commentary on the Srimad Bhagavata, Sanatana Gosvami explains that Sri Krishna went to visit Vraja for two months after the killing of Dantavakra and Salva, and thereafter concluded His Vrindavana pastimes, mounting on a chariot to reach Dvaraka again.
In regards to His age during the Rasa-lila, we find the following in the Krishna-book, chapter 29:
“It appears from the statements of Srimad-Bhagavatam that Krishna had to wait another year for such a moon [saradiya purnima] before enjoying the rasa dance with the gopis. At the age of seven years He lifted Govardhana Hill. Therefore the rasa dance took place during His eighth year.”
However, we should understand in this regard that the ages of Sri Sri Radha-Krishna and the gopis do not correspond to the regular growth of people; indeed, reading the Gita-govinda of Jayadeva, it is obvious that the Rasa-dance is not a pastime of eight-year old boys and girls, but rather that of adolescents. Sri Visvanatha Cakravartipada explains in the context of SB 10.45.3:
“The three years and four months that Lord Krishna stayed in Mahavana were the equivalent of five years for an ordinary child, and thus in that period He completed His kaumara stage of childhood. The period from then to the age of six years and eight months, during which He lived in Vrndavana, constitutes His paugaana stage. And the period from the age of six years and eight months through His tenth year, during which time He lived in Nandisvara [Nandagrama], constitutes His kaisora 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 Mathura, and on the fourteenth day thereafter He killed Kamsa. Thus He completed His kaisora 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 kisora.”