https://github.com/FiloSottile/age#ssh-keys
> (ssh-agent is not supported.)
There is no way it integrates with smartcards.
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.