Is there an age limit for Neidan?

Generally speaking, there is no age limit for Neidan. Male 16 or more, female 14 or more. Building a foundation in a hundred days is a spiritual treasure. In the past, people who were usually teenagers could skip this stage. However, at present, because of the temptation of all beings, the concept is relatively heavy, so even teenagers aged 13 or 14 need to build a foundation for a hundred days.