Is the creation date of accounts part of consensus? Can I query it within contracts? If so, how to do it?
The creationdate of an account is not stored in the chainbase. Accountnames are primarily stored in the user_resources_table which only contains accountname and resources. There's no extra table which stores accountname/creationdate-pairs so there's no way to query the creationdate of an account on-chain.
see here: eosio.system