I have a 25 year old cactus that is Browning at the base it is not soft to touch it is hard. It does have new growth andit is spiny at the very top. Please help I do not want to lose him!
It sounds like lignification, which means becoming woody (or corking). Many plants including cacti do this, and it can make the plant more rigid. It depends a bit on the cactus species, but I know Opuntias often do this when they get older.
I don't know what species your cactus is? If you want to be sure it is lignification, you might want to upload a picture.