Washington-based Allegheny Airlines yesterday became the first domestic airline to propose a reduction in fares for older citizens. Allegheny also proposed winter weekend discount faires for all customers and special discount to inaugurate a new Washington-Cincinnati service.

If approved by the Civil Aeronautics Board, after a formal petition is filed in the next week, Allegheny will permit persons 65 years of age and older to travel at a one-third discount from normal coach class fares, starting in January.

For a one-way coach flight on Allegheny between National Airport and Hartford, which now costs $46, the new "senior citizen" fare would be under $30.

The CAB in previous years has prohibited discount fares for specific population groups, such as students, as "discriminatory." But air cargo deregulation legislation signed by President Carter last week included a number of industry changes, one of which permits airlines to offer discounts for persons over 60 and the handicapped.

The Allegheny discounts would be available for most flights on a standby basis - which means if all the coach tickets haven't been sold. Qualified persons would buy an identification card in advance for $20 and the card would be valid for two years.

Travel would be permitted throughout the year, with the exception of certain peak holiday travel days.

Allegheny's other fare proposals, filed yesterday with the CAB:

Would seek to stimulate weekend traffic during the normally slow first quarter of the year, with a 50 per cent reduction from normal roundtrip fares, from Jan. 7 through March 19. The "Winter White Sale" discounts would apply if travelers begin trips on a Saturday and depart before 12:01 p.m. the next day, and

Promote travel on recently authorized nonstop service from Washington to Cincinnati, starting Jan. 7 and ending Feb. 7. A 50 per cent reduction would apply to passengers who begin and complete reoundtrip journeys on the same date.