Perhaps if we are dead set on making Citizenship an irrevocable right. Then we probably shouldn't be allowing people who've never lived in Canada and have no loyalty to Canada be citizens.
Yes, maybe age would be a defining criteria. I don't know how old the individual in this specific circumstance, but my suggestion is give people 3 years after their 18th birthday (ie. by age 21) to take up residency (minimum 6 months) to maintain their citizenship.