It should be just fine for a 13 year old. It was a little challenging near the top just because of the stone stairs but the view is absolutely worth it. I would plan for about 5 hours if you want to down some time at the peak.
No it's pretty easy there's a lot of hills but I did it when I was 4 and I'm a huge wimp so it should be fine it's like somewhere between 2 and 4 hours depending on speed and number of stops
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