Welcome to the forum!
My sweet Bell just turned 18 & takes such good care of beginners! I have only had her 4 years & plan to have her til it's time to put her in the ground. As a matter of fact, my dad has tried to get me to sell her while I can still make some money off her, but I refuse. She's my go-to girl for young, nervous or beginner riders because I know she is smart & willing, plus calm & forgiving, while at the same time, has enough energy.
My farrier says she has such an easy life & I take good enough care of her nutrition & feet she should live well into her 30s and be rideable for a long time still.
I'm a HUGE proponant of older horses for beginner riders.
For yours, try to discern what kind of previous life she has had. Was she used for eventing, heavy showing, reining, rodeos, etc? If so, 20 is a lot older than a sometimes-used mostly pet. A PPE is a really good idea, and make sure you're getting a fair price on a senior animal. It should be nowhere near the price of a comparable 10 year old. At the same time, remember that there are lots and lots of horses out there & if this isn't your horse, you will find a good match if you're patient. Make sure she has the qualities you're looking for, as well as the ability to do what you'd like to do. A horse that's been used for lazy trail rides probably won't make a great reining champ at this stage in life. A horse that has never been in a trailer or spent time around other horses may not adapt well to a heavy show circuit.
As others have said, be prepared for more expense and time in vet, feed & possibly farrier care.
Let us know what you decide & post pics of your new best friend, whoever that may be!