It's a combination of age of domain, age of pages, age of links, together with all the other
SEO parameters.
Certainly age helps, for example I can clearly see how sites are ahead of me in the rankings yet they display poor or no
SEO. I can only assume it's because they are several years older.
The weighting is heavy, but once you've gotten through about 2/3 year the disadvantage of young age diminishes.
Otherwise owners of old domains would be making a fortune selling them. I doubt google would favour an older site on age alone since some of the older sites haven't been touched for years and the pace of change of web media means newer sites are so much better.
The advantage of older sites is that have enjoyed page 1 ranking most of their life is that they have benefitted over time from many others finding them more easily and linking to them, or with them. So, the longer you are on page 1 the more chance of people linking to you - if you site is decent of course.