Almost certainly you do not have a case for discrimination. Age, length of service with a company and academic qualifications are not overriding factors in promotion decisions. However it is rather odd that you say every middle management position position is being lost and yet one person is being promoted at this time - I would have expected the people who are currently on the same job level as 'Associate Marketing Director' (whatever that means) and who are maybe being demoted might have the most to complain about.
Suggest you take the 'promise' of being promoted again when the economy picks up with a pinch of salt. Thousands upon thousands of people each year get promises of promotion yet it doesn't happen. Ask them to put it in writing - in the same letter as the confirmation of your 'new' demoted role. Use the explanation, if you are willing to say it, surely you can put it in writing. Should create a bit of a squirming action.