In this task, you're given a passage, further information available on a particular linked term from the statement, and a question. Your job is to generate the answer to the question by using the information provided. If there is no clear answer obtainable, output 'none'.
Q: Passage: John Robert Isner (born April 26, 1985) is an American professional tennis player who has been ranked as high as No. 8 in men's singles by the Association of Tennis Professionals (ATP). Considered one of the best servers ever to play on the ATP World Tour, Isner achieved his career-high singles ranking in July 2018 by virtue of his maiden Masters 1000 crown at the 2018 Miami Open and a semifinals appearance at the 2018 Wimbledon Championships. He has also twice reached the quarterfinals at the US Open in 2011 and 2018, the latter of which helped qualify him for his first ATP Finals appearance later that year. He currently has the second-most aces in the history of the ATP World Tour, having served 11,969 aces. At the 2010 Wimbledon Championships, he played the longest professional tennis match in history, defeating Nicolas Mahut in a total of 11 hours and 5 minutes, played over the course of three days.
Link Information: Mahut had excellent junior results, winning the Orange Bowl in 1999 and the Wimbledon Boys' Singles in 2000, turning professional the same year Question: How old was Mahut when he started playing professional tennis?
A:
Answer: 18