Well, did you bother to read the e-ticket ever ? If there is a such a clause it would be written on the ticket. Check any e-ticket, it says:
"One of the passenger booked on an E-ticket is required to present any of the identity cards noted below in original during the train journey and same will be accepted as a proof of identity failing which all the passengers will be treated as travelling without ticket and shall be dealt as per extant Railway Rules. Valid Ids:- Voter Identity Card / Passport / PAN Card / Driving License / Photo ID card issued by Central / State Govt /Student Identity Card with photograph issued by recognized School or College for their students...
more... /Nationalised Bank Passbook with photograph /Credit Cards issued by Banks with laminated photograph."
In addition for all tickets (e and paper) age proof is required for senior citizens.