Q: For some, Balarama’s age is a controversy. 1 year is mentioned in Brahma-vaivarta Purana, Krishna-janma-khanda 13.235.

                                                                  However, many Gaudiya vaisnavas say this contradicts other descriptions. Most acaryas seem to concur that the difference was just a few days.

                                                                  Which is it?
                                                                   
                                                                   
                                                                   
                                                                  In ISKCON our primary authority is Srimad Bhagavatam and its Bhaktivedanta purports.
                                                                   
                                                                  In Srila Prabhupada’s purport to 10.2.9 he writes “Balarama was only fifteen days older than Krishna.” That is the position I accept.
                                                                   
                                                                  This is confirmed in Jiva Goswami’s Commentary to 10.2.9, which reads –
                                                                  “Then, very soon after, the Lord says he will appear as the son of Devaki. Later it will be explained that Baladeva was only slightly older in birth.”
                                                                   
                                                                  Without providing the exact references, Bhanu Swami indicated that Vishwanath Chakravarti Thakur reasons that the developmental stages of infancy of the two children (crawling, walking, etc) were so similar, they must be the same age. So far I have been unable to locate those references.
                                                                   
                                                                  Garga Samhita describes Mother Rohini’s pregnancy being only for 5 days [Text 1:10:31]
                                                                   
                                                                  Sri Nanda said: “Who is this handsome boy, whose equal cannot be seen anywhere? How is it He was born after only five days (in the womb)? O great sage, please tell me.”
                                                                   
                                                                  All glories to Lord Balarama!