Metro Washington's 100,000 federal and military retirees probably will get their May cost-of-living adjustment (COLA) on time, even if the Reagan administration proposes delaying COLA entitlements for six months.

Officials say that any proposal to defer Social Security, federal and military benefit increases should not interfere with the catchup-with-inflation raise that goes into effect April 1 for U.S. retirees. That raise will show up in checks mailed for May delivery to three million civil service and military retirees or their survivors.

The amount of the raise will be determined by the Consumer Price Index for the month of December. The Labor Department will release that figure later today.

By law, federal and military retirees are supposed to get raises each year that match the increase in the cost of living--as measured by the CPI--for the previous year.

Based on the CPI increase from January to November of 1982, most federal retirees are due a COLA raise this year of at least 4.3 percent.

The actual May COLA adjustment for most retirees will probably be higher than 4.3 percent.

Younger retirees, however, will get less than a full COLA. In a budget-cutting exercise last year, Congress ruled that retirees who are under age 62 will get only 3.3 percent this year. To qualify for the full amount of the COLA, retirees must have reached their 62d birthday by March 1 of this year.

Retirees who are 62 or older, those who are retired on disability and those getting survivor benefits will get the full COLA raise.

Administration officials say that if Congress approves a deferral in entitlements as a cost-cutting move, it would not affect federal-military pensions until 1984. The 1984 raise for federal-military retirees currently is scheduled for June of that year. If deferral is approved by Congress, that raise would be delayed until December 1984.

But the next raise, which many retirees are counting on to partly offset big increases in health insurance premiums, is on schedule to be effective April 1 and payable in May checks.