From project page:

https://github.com/FiloSottile/age#ssh-keys

> (ssh-agent is not supported.)

There is no way it integrates with smartcards.

I guess you can use https://github.com/FiloSottile/yubikey-agent to store an ssh key on a yubikey, and then use age to encrypt to the ssh key.

Getting minisign to sign using a key in a ubikey is not supported out of the box, but should be possible in the same way yubikey-agent works (after all, that's what it does when you authenticate an ssh session using a yubikey).

You can ask Filippo Valsorda.