Is there any age limit for Aviva's Personal Accident plan's free child coverage?

Yes, there is. Here is the detail from the policy:

Your natural or legal child(ren) who is/are either Aged between 15 days and 18 years; or Aged between 19 and 25 years and

(i) not married; and

(ii) a full-time student in a recognised tertiary institution; and (iii) dependent on You for his or her maintenance support.

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