We all want youth, even when it comes to computers. As each nimbler model hits the market, owners of older desktop models fret and fidget.

Don't stew. Adapt.It's finally possible to update many older IBM PCs and compatibles with relative ease and safety. It's not inexpensive, but for IBM types it generally costs less than buying the newest machine. (In our book, you don't get enough increased performance in most updated Apples to justify the cost.)

Here are typical symptoms of the computer equivalent of hardened arteries:

* Your computer has too little temporary memory (RAM) inside to run some of the best new software.

* It has enough RAM memory for new software, but frequently runs short of it during complicated tasks.

* The permanent memory (disk space) is too small to hold all the data you want to store.

* The disk drives seem dipped in molasses whenever you try to find files or data, or try to save large files.

* The display screen doesn't show the graphics many newer programs demand.

* The screen displays text very slowly compared to newer computers.

If our age test made you even more eager to update, you're in luck. There's add-on equipment to fix each complaint, and the devices we tested perform very well. But go at it sensibly. Also, don't expect miracles.

For the average office computer, the biggest bang per buck comes from adding a new hard disk drive. If you've been running on floppy drives, the difference will be like switching from bicycle to Porsche. Even if you have an old 10-megabyte hard drive, a newer model's pace may pay for the upgrade - especially if you use a database or desktop publisher, which access disks much more often than other programs.

An old IBM PC floppy drive averages 240 milliseconds (ms) to find one piece of data. The original IBM XT hard drive does it in 90 ms. A new Priam add-on hard disk can cut the time to 20 ms. We added a $1000 40 M Priam disk to our 10M IBM XT for quadruple the storage space with three times faster access. Priam has similar speed booster disks for Apple IIs and Macs.

The next most important place to spread computerized Geritol is the internal memory. Early IBM PCs, XTs, and compatibles had 64 K or 128 K RAM. Many of today's programs require 640 K and run faster with more RAM than that. In general, the more program and file data your computer can store in RAM, the faster, better and more efficiently it does its work.

RAM memory comes on chips, usually in 16, 64, 256, or 1,000 K bundles. Like disk drives, chip speed varies. Early IBM PC and XT chips found or stored each bit of data in an average of 250 nanoseconds (ns). Today's memory chips in some high performance computers clock at less than 100 ns, almost triple the speed.

To get more memory, you can generally shove an add-on memory board into an empty slot of your older computer. But since the original memory path will still be most used by your programs, you'll fall very short of your speed potential unless you also replace the original chips with fast ones. Many people forget to do that.

Main processor chips get the most credit for giving computers their speed. You can replace your old, slow processor, but not by just pulling it out and sticking in another. First of all, the new one won't fit. Second, to get boosted speed, it needs new circuits.

What you can do is to buy a circuit board containing a fast chip and new circuits. Most of these boards also come with additional high-speed memory chips. After pulling out the old processor chip, you plug a special cord into the socket. The cord's other end plugs into the circuit board. Older boards use IBM AT type chips, 80286 (or 286) in jargon. New boards use the faster 80386 (or 386) chips.

We tested the Quadram Quad386XT board on an old IBM XT. It boosted some numbers-handling speeds tenfold. If you do a lot of numbers work, that alone can justify its $1,200 price.

This Quadram product also triples the amount of RAM memory your programs have available to work with. (This is true of most add-on speed enhancement boards.)

It's real easy to update an old computer's display capabilities. It's also expensive. An old monochrome monitor can't show graphics no matter what, so you may need a new monitor ($125 and up).