
Quoting
LandSurveyor
I would try to head off any upcoming adverse possession claim by immediately sending the neighbor a certified letter with signature receipt stating that it is your property, the fence is your property, and that he may continue to use your land up to the fence line UNTIL FURTHER NOTICE from you. Making the use permissive basically halts any claim on your land which he has not already acquired. It will give you time to decide what to do in the longer term.
Have your attorney do the letter if it can be done quickly. You are not looking at a huge cost to get a letter out.
When I read the words "about 21 years" I could just smell an adverse possession case coming down the road........