I agree that 12" tall JBP will be around a year old. Many seedlings will have just as good roots as many cuttings so I'd want to see the roots before paying high price hoping for excellent nebari
There are a number of different JBP cultivars - Thunderhead, Yatsubusa, are just a couple - but those can't be propagated as seedlings though some cam be grown as cuttings from an authentic parent tree.
Some of the cultivars are OK for bonsai, others are more difficult than straight JBP.