I have seen beds that were built to a height consistent with the wheelchairs. There was an awful lot of soil used to fill them and they used concrete block or decorative stone to hold the outsides of the bed together.

Things cannot be planted very far back in the bed - because the elderly will not be able to reach to the back part of the bed. An island is an ideal shape - so they can work on all sides of the garden.

Hope this was of help. Visit All The Web and enter in raised beds, elderly, or other key words - you should get some website for reference.

Here is a link that might be useful: All The Web

Good article appeared in Birds and Blooms magazine this month

How about a couple "table-top" raised beds? If you built the sides and bottom strong enough to hold 6" or so of soil, it might work. Good luck!