Mine only miss their litter tray occasionally and for the following reasons - 1) When Athena came along - Zeus sprayed her and "his" territory - has there been a new rabbit introduced or the smell of a new animal?
2) Athena won't go in a dirty litter tray, if it has more than two days old she won't use it - fussy little madam then deliberately squats down in the middle of their rug to make a statement I presume!
3) If they are suddenly frightened
4) They have been away from their litter tray too long and are so desperate they don't make it back (when Zeus has been on the sofa he sometimes can't hold it for long)
5) Sexually maturing males will spray to mark their territory (is Mikey done?)
1 is not old for a bunny so it is unlikely to be age causing him to miss unless by age you mean going into adolescence - it might simply be a one off accident. If it keeps happening or you notice constant dribbling (like he as no control) then it would be worth taking him in for a check up!
Caz