I've heard that G considers the domain age as an important SEO factor, as that works filtering some spammy domains. How important do you think this is?

Domain age is also a bg factor to decide ranking and PR. Google loves the old sites.

Yes,of course google gives more credits to the old age domain and keyword included domain.

yes, domain age is really a major factor for Google. If your site is old then there is chances th site ranked well in the search engine rather than the new sites.