The Rav Mibartenura records in his letter how they celebrated Lag Baomer in Meron. He was around 1500.
The Chasam Sofer having heard of people tossing clothing into the fire condemed it as Bal Tash’chis. Those Poskim who lived in Eretz Yisroel and who witnessed the Simcha had a very different look at it, and responded to that Teshuva.
The Arizal took part in the celebrations, as told by Reb Chaim Vittal. He even revealed to a Talmid that this Talmid will be punished for ruining the Simcha by saying Nachem.
The day was always considered special. What is new is the association with Rebbi Shimon and the degree to which it is celebrated. The Maharal in Nesiv Hatorah writes about the significance of the 18th of Iyar.