Hi Diane
When a person reaches state pension age - their mobility award cannot be increased. So if someone has no mobility award they cannot claim after state pension age. If they have a standard mobility award 11 points or less they cannot move to enhanced after reaching state pension age. If they have an enhanced award when they reach stage pension age - 12 points or more - they can continue to receive their enhanced mobility award.
You said that you have spent the last 2 years helping your uncle get enhanced rate mobility. It depends on when he reached state pension age and whether he was awarded enhanced mobility before or after. If you started the process before he reached state pension age - and yet you have been told by the DWP he is not eligible, I would put in a formal complain and argue with the DWP that it's not your uncle's fault that it has taken so long to process (if it has).
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems