So, is it true that Radha is elder than Krishna during their marriage?
Radha may not be elder than Krishna during the marriage between them. It is not true that Radha is fourteen years elder than Krishna during their marriage time.
The reason is that Krishna takes the form of teen-age youth before wedding moments
Then the Supreme Personality of Godhead became a teen-age youth splendid as a dark cloud, dressed in yellow garments, decorated with the Kaustubha gem, holding a flute, and more enchanting than a great host of Kamadevas.
Krishna married Radha after taking the form of teen-age youth only
Brahma performed the wedding-ceremony of Lord Hari and Shri Radhika. He had Them circumambulate the sacred fire and bow down before it, and then Brahma, the knower of the Vedas, recited the seven mantras.
After the marriage, Krishna takes his original infant form
As Radha was about to decorate Him, Lord Krishna suddenly abandoned His teen-age form and again became an infant. He became an infant crawling on the ground and crying, just as Nanda had given Him before.
Thus, according to the scripture Garga Samhita, lord Krishna changes his form from infant to a teen-age youth before marriage and marriaged Radha in his teen-age youth form and then again took his original infant form after marrying Radha.
Hence it is false that Radha is fourteen years elder than Krishna during their marriage time.