how important is domain age?

Older ones has splendid backlink network. This profile usually makes it harder for fresh domains to outrank them.

It's one of the factors but not the main one surely. If we compare two domains one 1 month old and one 5 years old then yes I agree it's a big difference. If say one is 2 years old and one is 10 years old then it's not even among top 5 factors I guess. Quality backlinks are way more important IMHO.