Taḥammul is acquiring a hadith through a recognized mode of transmission; adāʾ is delivering that report to others. The qualifications for the two acts are not identical.
At the time of hearing, discernment and an adequate capacity to understand and retain the report matter more than an inflexible numerical age. Children attended hadith sessions, and their names were sometimes recorded, but physical presence alone does not establish valid acquisition.
A discerning child who truly heard and retained a report may narrate it after maturity, subject to the conditions of performance. The research question is whether reception actually occurred and was adequately preserved.