Its not age so much as attitude. When I was 14 I was very independent, but also very immature. I would have probably killed myself if I had either gotten the motorcycle I wanted so badly or if I had seen some of the Parkour videos out there!
What's most important is your patience, and willingness to train correctly and safely. There is NO need to ever go on a roof, and nothing about being on a roof that makes you any better at Parkour or Freerunning. What makes people good is practice, consistent, safe practice. Going big leads to injuries which leads to less consistent practice. Going too far outside your comfort zone leads to hesitation, leads to injuries, leads to less practice. Parkour and Freerunning are not about balls or guts, they are about honing your abilities so that more things are within your realm of safe possibilities. Even then there is a risk of danger, "even monkeys fall from trees".
So, I can't tell anyone a specific age is right or wrong, but here's a guideline: if you are disciplined (yes, Parkour is a discipline, funny how bad words and good words are the same words
) enough to train for three hours a week by setting up a goal or plan and sticking to it without jumping off anything over your own chest height for a month straight, then you are suited for Parkour and Freerunning safely and will probably get good at it. On he other hand, if you go out to train precisions, and ten minutes later you're jumping off 6' walls and rolling before you knowhow and doing running jumps, then Parkour and Freerunning are not for you, you will only end up hurt.
Don't go out training alone, and if you're less than 18 don't go meet "people from the internet" without at least a few friends or even a parent. There are a lot of other pieces of advice about starting to train that aren't included in this article but exist in others, like whether to start in a gymnastics gym with padding, training alone or with friends, etc.