My elderly bunnies have always acted bright as a button and have been hard to pick apart from the younger ones. Tubby was nearly 12 when he died and you could tell he was an old bunny physically (he was quite frail and also his siamese smoke pearl fur had taken on the colour of the dominant gene and gone sable, so he was brown around his muzzle).
He was so full of life though and had the biggest spirit for such a small, old bunny. Even on the day he was PTS he still had bags of spirit. We did everything wrong with him though, and I suspect, had things been done right, then he would have not had struggled so much physically.