The English language article on geisha says
Apprentice geisha are called maiko (舞子 or 舞妓), literally "dance child") or hangyoku (半玉), "half-jewel" (meaning that they are paid half of the wage of a full geisha), or by the more generic term o-shaku (御酌), literally "one who pours (alcohol)".
and later on says
The only modern maiko that can apprentice before the age of eighteen are in Kyoto. So on average, Tokyo hangyoku (who typically begin at 18) are slightly older than their Kyoto counterparts (who usually start at 15).
Is 舞妓 used in all of Japan, or does the term for apprentice geisha vary from dialect to dialect, just like the word for geisha can be 芸者 or 芸子 depending on dialect? Alternatively, are 舞妓 and 半玉 now considered distinct terms describing different concepts?