A timeless picture has a spiritual quality to it. A timeless story has the same characteristic. There is a spiritual component to a work that remains popular. They could easily be us. Timeless characters react to a situation in a unique way, perhaps is a way that we had not thought of before.
Timeless characters are logical yet emotional, rational yet spontaneous.
And we don't forget them. That is why I self-published my books. I could not wait four years for the issue of disaster preparedness in the schools to come to the market place. Join critique groups. Go to seminars.