Sort of. For their own domestic airlines, nations can set whatever age limit they like, or none at all.

In order to fly internationally, under ICAO rules, they need to comply with ICAO standards... and they ALSO need to allow ICAO-compliant airlines to operate in their own territory.

So if ICAO makes the change, everyone will be able to do it, and they will also have to allow everyone else to do it. They can still limit their own pilots to a lower age if they want to. IIRC a few countries still have an age limit below 65, but they have to allow us to fly there. Of course nobody "has" to subscribe to ICAO, but the vast majority do, even N. Korea.

The ICAO model is important, because it applies to countless other regulatory aspects, pilot age is just one of many. It allows reciprocal access, without having to comply with every nation's specific regulatory details. Instead it provides a regulatory baseline, which everybody can agree to. Or not, if they don't want any part of it.

You can also have specific bilateral agreements between nations, "side letters" if you will. Hypothetically, two countries could agree to a common age limit which differed from ICAO's, or something like allowing a common language, such as two Spanish-speaking countries allowing the use of Spanish on their mutual international ops. The vast majority of bilats address air service access rights, ie degrees of freedom and not so much operational regs. Most folks just stick with ICAO.