Metnally disabled dependent son above 18 years is covered under esi?

THE EMPLOYEES STATE INSURANCE ACT, 1948

[Act No. 34 of 1948]1

2.  Definitions.

[(11)     family means all or any of the following relatives of an insured person, namely :

(iv)   a child who is infirm by reason of any physical or mental abnormality or injury and is wholly dependent on the earnings of the insured person, so long as the infirmity continues ;

https://esic.nic.in/Publications/ESIAct1948Amendedupto010610.htm

https://www.esic.nic.in/Tender/ESIAct1948Amendedupto010610.pdf

Sorry, he crossed 21 years. 

a child who is infirm by reason of any physical or mental abnormality or injury and is wholly dependent on the earnings of the insured person, so long as the infirmity continues ;

above 21 years depenednents also comes under child? please clarify?

Your establishment must be having its Legal cell/Law firm specializng in labor matters.

Refer the matter to them for proper legal advise in writing.