I cut taproot of Astrophytum asterias when seedlings are 2 years old.
The season of the cutting is mid of August and rooting is started in mid of September.
Asterias like a very hot condition. If you fed a prenty of water in hot summer, asterias seedlings grow very well but it will be increased the chance of rotting from the taproot.
I use same method for other Astrophytums but not for Ariocarpus and Copiapoa.
2 years old miracle kabuto, before cutting tap-roots
After cutting off tap roots
3 years old seedling, 1 year later after cutting taproot, more capillary roots