My fairy castle is the one I've had the longest (a little over a year), but I'm curious as to whether or not an age can be approximated for my S. truncata that I got last summer, based on the corkiness of the base.
@ata1515: The mammmillaria you´re showing should be at least 3-4 years
@skyclan cat: Schlumbergera are often propagated by taking cuttings of the same clone. If you inquire when that special seedling of the plant came to live you will know the actual age of your plant (Which will probably be much older than the cork on the stem )
edit : I don´t have any schlumvbergera specimen in my collection so it´s hard to say for me at which point of their life they get to be corky at the basal segments.