That's a difficult question to answer. All known shinigami from Rukongai were young. Kenpachi, Ikkaku and Yumichika are the only shinigami who were adults when they joined the Gotei 13. However, it must be noted that all three had Zanpakutos before they joined while Rukia and Renji (especially Rukia, who is a kido expert) only had limited kido like abilities while in Rokungai. Other children, Gin, Rangiku, Hisagi and Hitsugaya, also have not been shown to possess zanpakutos while they were just children in Rukongai.