I know the age of your site gives credibility to SE. Is it the age of the domain ? ie when the domain was registered ? If this option would it count if it was first used for a website but then completely changed the layout, would the site age still count from the date the domain was registered ?
as far as i know SE count domain age by the age of your domain name and it is okey if you change your web layout but i am not sure about the content if you change your content. but as far as i know it is okey as long as the same or related content
Not sure. I am a beleiver that age helps a website rank better though. I'll be anxious to see input as to if major changes happened to the site though.