You would have to look at their first posts. “Accounts” are private keys so the concept of age would be when it starts sending out activity to relays. Use something like https://www.nostr.guru/ to figure that out.

this is where NIP-05 comes into play. the real #[6] could register a NIP-05 ID against a domain that we all know that he owns. this is why NIP-05 is important.

you DON’T want that

Account ageism is a thing on Twitter and it’s terrible for privacy