I am wondering have you had this clown for a year and a half and it has been sleeping in the same spot? Has it ever tried to host anything like a leather, mushroom or polyps? My brother's hosts a feather duster.
Mine chose a corrner to sleep in when they were young and pretty much slept there until they started hosting the mushroom. My new onyx chooses to sleep in a corrner near the surface, sometimes I check to see if it still alive, very wierd, usually they will choose a spot at the bottom on the sand. When not hosting they seem to roam all around the tank but once they are hosting they stay very close, darting out to grab food then right back again. Your fish is absolutely not too old or I think you mean set in it's ways to change it's preferred area, just give it time, remember mine took a year to find it's anemone(s) I have just rescentely sold my very testy maroon, it hosted very quickly, I just hated getting bit every time I entered the tank. I'd just give it some time it may host tomorrow or a year form tomorrow. If you choose you can always sell it and get a maroon but ther is no gurantee that the maroon will host for you either. : )