Thank you.
Posted.

It starts from 0. He\she will only start growing from the day it was acquired. Please check, "Dragon Vale FAQs" 


If I have more than one dragons, how does XP allocation works?
Answer: All dragons you own will benefit from the XP you earned, proactively. This means that a newly tamed dragon will get XP from your task completions starting from the day it was bought. For instance, you have 1 Nature Dragon, 1 Light Dragon and 1 Fairy Dragon. When you earned 100 XP from completing a task, all three get 100 XP each.