Sadhana and neurological disorders

Namaste,
 
Dharma Sindhu says that gAyatri mantra can be imparted to napuMsaka, 
blind, deaf & dumb, dwarf, vikalAnga.  Which implies that all these can undergo
upanayana samskAra.  
 
However, this samskAra has been denied to mentally retarted persons.  It further says that
even though the mentally retarted persons are vaidika karma anadhikAriS, there is no 
bhraSTatva on their part.  But, their male offsprings can be imparted the samskAra
provided they don't have the mental retardness.  
 
This is just an example from Dharma Sindhu regarding the *dvijatva siddhi*.
 
regs,
sriram