Hi Monika,
I agree that this is probably age related. There is no reason to stop him from doing this unless he is pooping in the dish. In that case, you could get a second dish to leave empty for him to rest on at night. You might also look into a corner shelf for pet birds – there are several types available in wood or metal. Metal is easier to disinfect, but wood might be more comfortable. I would place it where he is wanting to roost at night – this is generally a higher place in the cage. There are also flat perches available that might be more comfortable for him. It is still important for him to have a variety of perches to climb on during the day, and this will help keep his feet from getting too stiff or crippled.
I am not sure if you have him on a senior diet, but you might try giving him our Nutri-Berries for Senior birds. These contain special natural ingredients that help with inflammation and arthritis as well as other issues associated with senior birds.
Senior Bird Nutri-Berries for Parakeet & Cockatiel
You have done a great job of taking care of him for him to reach an age that many cockatiels do not. Keep up the great care and love.
Thank you for asking Lafeber,
Brenda