It's not a replacement: age is purely a file encryption tool. GPG is encryption, signing services, key management, web-of-trust, smartcard compat, and all sorts of bells and whistles. If you're just encrypting files and don't care about any of the other stuff it's probably a solid bet though.
The argument for people using age to replace GnuPG (PGP really) is that there are better signing tools that you really should be using (minisign/signify) and PGP should not be used for the web-of-trust anymore anyway. There are many good arguments against PGP for all of these things.
