Same Time Next Year, Hilton Playhouse. June 9-19. THE bell-bottoms and love beads have come back, but what about the language and mores that went with them? Did people really say ''peace, man'' and think love was for free? How ironic the Age of Aquarius seems now and how well Bernard Slade captured it in Same Time Next Year - at least in its American context. With nostalgia for the past in full flood, the timing couldn't be better for a revival of this bitter-sweet comedy about an adulterous affair spanning a quarter of a century, and director Robin Herford has chosen his couple well. Their accents may be suspect, but John Duttine and Mel Martin ring true as lovers George and Doris. If a hollow note intruded from time to time on Wednesday it was understandable. The previous night they had played in Kuala Lumpur, then it was up at daybreak to catch their flight to Hongkong and not a moment's rest before they opened at the Hilton. The tension and exhaustion did show, especially in the first half, but these are pros and they delivered. The full brunt arrived in Act Two: that moment of brutal tragedy in this episodic two-hander which seems to come out of nowhere. That's how it should be in Same Time Next Year and that's how it was as Duttine and Martin made that rare transition: no longer characters, but real people clinging to each other in terrible grief. There were other rewards - Martin as the late-blooming flowerchild, Duttine's wicked portrayal of the 70s George - as throughout it all you were conscious of those betrayed spouses, Helen and Harry. It was also apparent that despite the passage of time, the cloth on the piano, the bedside phone and the picture on the wall never changed, and Duttine's accent went veering off to a Kosher deli in the Bronx in the final scene, but the flaws were easily forgiven. Not a tour de force, perhaps, but a solid, entertaining enactment of a play that has worn remarkably well and has plenty to say to today's generation.