Nothing special happened to Ciel at the age of 12 in the original Tsukihime. It's possible that the remake made it so that
Roa took over when she was 12 rather than 16. But her body clearly still aged past that point before he was separated from her.
https://i.imgur.com/lfI7zGE.jpg https://i.imgur.com/tgLNUuX.jpg
Come to the (un)official GameFAQs Touhou board. http://www.gamefaqs.com/boards/1110-