Nowaday, with modern transport vehicles, you dont have worried about the old. They can use flight, train with luxury bed. However, I would like to recommend to spent days to visit some famous cities like Hanoi, Hue, Hoi An, Da Nang, Ho Chi Minh, and play at some island or visit the beach instead of hiking or trekking.
We can make the tour suitable for them, if they still wanna discover the mountainous areas at the north!