YOU'RE ONLY OLD ONCE! By Dr. Seuss. Random House. Unpaginated. $9.95; All ages.

DR. SEUSS HAS written yet another terrific book, his 45th, but this one isn't really just for children (though the 6-year- old at our house thinks it's great). The book jacket suggests "you buy a copy for your child now and you give it to him on his 70th birthday." Because who but a 70-year-old can truly appreciate Dr. Seuss' gentle jabs about being old?

With his most realistic drawings ever, Dr. Seuss follows our nameless hero -- a kindly bald man with a big white mustache -- through ridiculous examinations at the Golden Years Clinic on Century Square (he's there "for Spleen Readjustment and Muffler Repair"). Poor old guy. He's "questionnaired" by the "Quiz-Docs," ogled by the Oglers (who "have blossomed like roses in May/ And silently, grimly, they ogle away"), and placed on a bed of nails by Dr. Van Ness, the expert on stress.

In between examinations -- like a hearing test that assaults the patient with sounds of a grandfather clock, a couple of braying Seuss creatures, and a few horns and drums coming out of the walls -- the old man sits in the waiting room. He reads National Geographic and talks to Norval, the Clinic Fish (a perfect touch; doesn't every clinic have an aquarium in the waiting room?). Norval is the spitting image of the pompous pet in The Cat in the Hat, but here he, like the others, has grown older, wrinklier, and much more forgiving.

The clinic is cute, but its realism is a clue that Seuss really wrote this book for adults -- the older the better. Fortunately, he couldn't resist a few fanciful touches, like the signs and arrows directing patients to Seussian departments like Optoglymics, Dermoglymics, and Nootronics, and a couple of cacti that resemble the plants in the Lorax stories.

My favorite department is where we find the Wuff-Whiffer, a "Diet-Devising Computerized Sniffer." As an array of foods pass under your nose, the Wuff-Whiffer tells sadistic Dr. Von Eiffel just which are your favorites. "And when that guy finds out/ what you like, you can bet it/ won't be on your diet./ From here on, forget it!"

My 6-year-old preferred the Pill Drill, an elaborate, lengthy chant to help our old friend remember what pills he takes when. He takes more than a dozen a day. ("This small white pill is what I munch/ at breakfast and right after lunch. I take the pill that's kelly green/ before each meal and in between . . .") I think my 85-year-old grandmother would appreciate the chant even more, and would feel a special chill at its last line: "This long flat one is what I take/ if I should die before I wake."

The book is lighthearted, silly, but with an undertone of complaint. Being old is sometimes tough, isn't it, the 82-ear-old Seuss seems to be saying. Still, when the book ends we're reminded that, irksome as it may be, old age is better than the alternative. On the last page our aged friend is natty once more -- he's retrieved his gold suit and pink bow tie -- and Seuss sends him off with a cheery farewell: "you're in pretty good shape/ for the shape you are in!" We should all be lucky enough to get old the way this man, and Dr. Seuss himself, have gotten old.