I think it depends on the gamer’s life situation than their age. If they’re 40 years old, are self sufficient and making progress towards their real life goals then I don’t think most people would consider them wasting time if they’re playing video games in their down time. If they’re 16 and spend a disproportionate amount of time playing video games while neglecting the rest of their life (school, work, relationships, productive hobbies, etc) then most people would probably consider them to be wasting their time.

I’d say that the age it becomes unacceptable to spend many hours every time playing video games is probably around 16 or so, as that’s when most kids start to take on more responsibility and start saving for their future. A 14 year old spending 12 hours a day playing games every day over school break isn’t a big deal. A 16 year old doing the same might be neglecting other aspects of their life pretty heavily if they do the same. An 18 year old most definitely is. It all depends on their goals I guess though. Some adults make a living by streaming themselves playing video games many hours everyday, so for them it’s not wasted time. This question is very context dependent IMO.

In moderation I don’t think there is an age where gaming suddenly becomes a waste of time. Everyone has some unproductive vice of some sort, so if someone’s enjoying the time they’re wasting on video games then they’re not really wasting their time.