When can a PC during creation take Art talents. If I understand it correctly, a PC can take Art talents instead of regular talents as soon as the PC has the Mystic talent.
A sidebar at the beginning of Chapter 17 has your answer: " These talents, and the Art talent
trees later, are all regarded as Mysticism talents." so after your character has gained the Mystic Talent and they can grab Mysticism Talents, then they can get Arts Talents as well.