Erm...at the moment I don't think it is actually possible. Avalon at the moment exist in Spiritual state rather than physical state in Fate/Stay Night where it reside inside the owner of Excalibur, unless you kill the owner of Excalibur, Avalon carry on providing indestructible protection for its owner. However, I believe if the owner is corrupted by evil (Saber Alter), the protection of Avalon will no longer be in effect. (but I'm not 100% sure on that)
Also, aging doesn't work, since Avalon also provide hasten regeneration rate, so owner will also be granted eternal youth, you have to somehow nullify Avalon's effect first before you can kill the owner, which in another term Avalon technically provides indestructible protection to its owner unless the owner will otherwise