I'm quite surprised that age's interface doesn't provide for signing of messages.
https://age-encryption.org/v1 For between participants, the payload is self authenticating by nature of the crypto... you received it, from somewhere, that may also have included some more identity inside, it decrypted, done. For standalone clearsign maybe user could encrypt a hash of the signable content, to a separate publicly published age secret keypair that they gave provenance, that anyone could then decrypt, but that is convoluted, and is in conflict with standard of teaching people to never publish any secret key. Much better to use pgp, signify, minisign or whatever else for public sigs. As for age, people can play with this key which will expire and be destroyed on or before 2022-04-30... age19nr5khmhtwe0jp0f2yvh2cevsqaev5tjkq0zw5t5ruy2uvrgfsysl7r6ch