Yes, ArangoDB is young (6 years) in comparison to PostgreSQL (30 years).
Yes, PostgreSQL is a phantastic database with an amazing open source community and is not going away any time soon.
Yes, PostgreSQL is a good choice for a project in which you need a relational single server database.
However, ArangoDB actually has a different value proposition, so in a sense, it is not a direct competitor.
ArangoDB is native multi-model, which means it is a document store (JSON), a graph database and a key/value store, all in one engine and with a uniform query language which supports all three data models and allows to mix them, even in a single query.
Furthermore, ArangoDB is designed as a fault-tolerant distributed and scalable system.
Finally, we do our best to make this distributed system devops friendly with good tooling and k8s integration.
Last but not least, ArangoDB is backed by a company which offers professional support.
Therefore any well informed decision for a project needs to look at the value propositions and capabilities and not only at the age and experience, which is of course a big argument, since people are - rightfully so - conservative with their databases.
What is the consistency model and have you validated that it actually works as designed (for example with Jepsen)? I didn't find anything detailed on your website.