Yes, there isn't a minimum age or any other age or health restriction to visit the Citadel. In fact, even children can visit Machu Picchu because it is not very hard or risky. The visit takes in general 3 hours and it is done following a marked fixed path but, besides some places where the access is forbidden, the visitor can walk around and visit almost the entire Citadel.
At the beginning of the visit, some people would like to visit the mountain nearby which dominates the Citadel, calledWayna Picchu. We think it is a must and the ascension is worth it. The visit of Machu Picchu allows you to walk around the streets, see the sites, the houses, the monuments, just from right there. The visit of Wayna Picchu allows you to have a great overview of the whole Citadel and the view is really awesome. You can also go to Machu Picchu Mountain to have a fantastic view of both the Citadel and Wayna Picchu. The access is a bit difficult because you have to climb high stone stairs to get to the top, it could be hard for the knees.
Regarding the access to the citadel, there is not much difficulty: there is no escalade, not dangerous places, just some difficult passages. To sum up, this visit is open to everybody, it takes half a day and can be done with or without visiting Wayna Picchu or the Mountain. However, people with mobility issues can visit Machu Picchu but maybe not the entire site. Wheel chairs are not allowed, they cannot circulate through stone paths easily.