Many travellers in their 70s and 80s not only visit MP, some even do the Inca Trail.
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Be sure to schedule an extra down day in Cuzco or the Sacred Valley when you first arrive, as that will allow your group to acclimatize before heading off to hike around Machu Picchu (which is actually a lower altitude, but can still be a shock if coming from sea level).
There are a lot of steps at the ruins, but if they have average fitness for their age they should be completely fine.
Just got back from MP and saw many in their 70s walking in MP. The first part of the hike is the hardest. I would estimate a elevation gain of maybe 300 ft, 100m. This is done over many switchbacks with part of the trail like a ramp and other parts like steps. Just take your time and rest. My wife and I are in our 60s and we made it up in 20 minutes the first day and it felt even quicker the second. Once you make it up the intial climb you are at an overlook with the iconic view of MP and everything from there on is downhill
I did see a lot of older people with walking sticks so you may consider bringing those if steps are a problem.
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