Oh wow. Only thing I can think of is for everyone to take a mandatory computer basics class/workshop/training session. Of course, even if you don't single her out, everyone will probably know you're doing it for her. But having everyone do the extra training can't hurt. Spend a nice long training session pretending to ring up various transactions at the POS. "Suzy A. is buying this bracelet sku and is paying with a personal check". "Carl P. is buying these two pair of stud earrings and is paying $500 on his American Express and the rest on his Visa debit card". I've worked with many older high-selling salespeople who cannot work the computer, and I know that I personally get extremely frustrated...especially the seventh, fiftieth, or hundredth time you show them, since it's so easy for me. The only other solution is to have an "Office" employee like some larger chain stores do. The office employee's main duty is to ring up, gift wrap, place special orders, and basically be Master of the Computer. However, since you have a smaller business, I don't know...I hope things turn out well for you.

It seems that you have two problems. One is the aging thing--a whole other topic, and I know a very difficult one. The other problem is this particular employee's inability to do the computer work. After that much time I think she is just expecting others to bail her out. I'd have a talk, in which you make it absolutely clear that it is necessary that she do this. "How can I help you learn to do the computer work that is necessary for your job?" I think if she realizes that you really expect her to do it herself, she will. Be prepared to offer a paid class for her to take. Make it clear that she is to take the class or learn it on her own, but that you will be following up.
Discussing this might also bring things to a head as far as aging problems. It might open the door to discussion of retirement or a change of duties. I know it is difficult but your customers cannot be treated this way. Aging is hard for bosses to deal with. We have a staff member with things happening, and we are just going through the older generation owner of our business hanging on way too long. You must take care of the situation. You have my sympathies and support. Treat the computer thing as a thing of its own regardless of aging. It's the ability to do the job.

Delia is right--you have to address the computer issue as a part of the job that she is underperforming, period. I don't think you can legally bring up the age issue at all, so don't go there. But you can make it clear that the computer is part of the job and if she isn't doing it, she isn't doing the job she's being paid to do. Yes, of course offer classes and training for her--no point in making everyone do the same and paying for it if they're performing ok on the computer. I'd worry less about singling her out for class than about covering yourself legally and making sure she is being given every opportunity to get trained and up to speed. Finally, if she's THAT great of a salesperson, MJB's idea might be worth considering--but only if she's so far ahead of the rest of the team in selling that you can justify it to the team as to why she gets that preferential treatment. Otherwise you're in for a mutiny at some point.