Shell International Marine, a subsidiary of the Royal Dutch/Shell Group of companies, has bought four tankers from the Onassis Group.

The ships were built between 1972 and 1977, but Shell defended its decision to buy the rather elderly vessels. The company issued a statement saying, ''the technical standard and integrity of a vessel are determined by the standards to which it was built and by its maintenance and are not dependent upon age."Shell has not disclosed the price of the four vessels, acquired from the trustees of the estate of the late Christina Onassis and the Onassis Group, but a rumored figure of $125 million was circulating in the London tanker market Tuesday.

The 265,000-deadweight-ton Olympic Banner and Olympic Brilliance, the 270,000-dwt. Aristotle S. Onassis and the 273,000-dwt. Alexander S. Onassis were formerly operated by Olympic Maritime SA, part of the Onassis group. No one was available at Olympic's London agent Tuesday to comment on the sale.

A Shell spokesman said the Onassis group had a good reputation for running a well-managed fleet, a claim backed up by tanker market sources. The spokesman added that Shell pays very close attention to training and the quality of a ship's crew, and undertakes its own inspections, which include checking the qualifications of the crew before chartering a ship.

A survey on extending the life of older ships published this week by Drewry Shipping Consultants Ltd. says many vessels will find it difficult, if not impossible, to meet the higher technical standards that charterers, and particularly major oil companies, now demand.

The report also agrees that the condition of a ship depends very much on vessel ownership, and that the trend toward management companies running ships on behalf of owners whose only interest is "asset play" has led to a deterioration in the quality of many ships.