Had he opted to use a little bottle of coloring for his locks, evenMethuselah might have shed a few years in appearance.

And it`s what`s on top that counts, too, for the Mustang, Ford Motor Co.` s ancient warrior, which as a convertible seeks to help its owners find thefountain of youth. The droptop does melt the years-the Mustang`s and those of the person behind the wheel.

Lower the top, roll down the windows and you tend to forget the Mustanghas been around longer than most of Ford`s employees. About 26,059 people haveshort memories, because that`s how many bought a Mustang convertible in 1990. Sales will fall a few thousand shy of that in 1991, thanks to the recession.

We test drove the Mustang GT, the top-of-the-line performance model inthe ragtop series, with its exceptionally quick 5-liter, 225-horsepower, V-8. With that power plant the GT is capable of swift spurts, but the 225 horseswill need to stop rather frequently at the ol` fuel trough to quench thethirst from such activity.

The 5-liter V-8 teamed with automatic in the test car we drove is ratedat 18 miles per gallon city/25 m.p.g. highway. But those ratings seem to beoptimistic.

Any convertible is fun to drive, especially between dusk and darkness.Drop the top and cruise. Enjoy the freedom. When the chill starts to bite yourskin, roll up the windows or turn on the heater or both-but keep the top down. The Mustang is fun, but age has caught up with it. There`s a power top to make raising and lowering the vinyl simple, but the rear window is plastic,not longer-lasting glass.

When the top is down, there`s a vinyl boot to cover the hardware. But the top doesn`t slip completely into the body cavity and there is a bulge with theboot on. Today`s clean look calls for the top to lower far into the body soyou can close a plastic tonneau over the top and hide it totally.

Ford has said a new Mustang is coming, probably in 1994 as a `95 model.Then the convertible will catch up with the `90s technology in rival ragtops, and the body will get new sheet metal.

The GT we drove starts at $19,992. A special value group including cargotie-down net, front floor mats, cruise control, AM/FM stereo with cassette andclock and air conditioning added $1,449; automatic added $595 and leatherseats $489. The sticker topped the $22,500 mark before adding $440 for freightbut subtracting $700 for option discounts.

A while back, we had the pleasure of driving the midsize `91 OldsmobileCutlass Supreme convertible. The sticker was roughly $1,500 more than that on the subcompact Mustang. In addition to more standard equipment, the Cutlassalso features more contemporary styling, a V-6 with a less voracious appetite for fuel and a back seat that holds two adults unlike the Mustang`s, whichholds two bags of groceries but not the people to hold them.

The Mustang GT convertible makes the Olds Cutlass Supreme convertib lelook even better for the money.