Yes, in Australia Tough Mudder 3-5 mile (5-8k), and 10+ mile (15k) participants must be at least 14 years old.
Any participants under the age of 18 taking part in the Tough Mudder 3-5 mile (5-8K), Tough Mudder 10 + miles (15+K) will need to be accompanied on course by a chaperone who is 18+ years old.