When did Jaleel White get too old for the role?
I think around season 5. You could tell he was struggling to keep up the Urkel nasal voice, as he was becoming a man with a deep voice. His voice sounded forced every time he spoke. He also became taller than Carl, and acquired a leaner, manlier face.
At that point, the Urkel character started losing its mystique. Jaleel was around 17-18 in season 5.