I was told that for a big upcoming event that's seeing 100's of cars heading to Bamako, the Senegalese decided to levy an additional tax on cars older than 5 years. I have no idea if this is just for this special event, or if it applies to everyone.
Copy paste from this event's organizers:
Senegal's only problem was an obscure law that doesn't permit
vehicles older than 5 years to enter the country. During
the summer we negotiated an arrangement with the local
authorities that part of our permit will include a waiver
for cars older than 5 years.
This morning creative Senegalese customs officials said
that they don't honor [this] arrangement.
However, they are willing to allow cars older than 5
years for x EUR per vehicle. (Normally this
rate is x EUR).