The phrase "developing stage" refers to a time when something or someone is growing, changing, and progressing towards a more advanced state or level. It usually implies that the thing or person is not yet fully developed or matured. Full definition
Those who ate the most red and processed meats had heightened risks of developing any stage of prostate cancer, or advanced cancer in particular. (criticalillnessinsuranceinfo.org)
If the analysis here is correct, a new rhetoric is still in developing stages. (religion-online.org)
His brain, bones, organs, lungs and eyes are at developing stage and they have extra nutritional needs. (doctor-info.website)