The age field for private servers can be wonky. As I understand it there’s specific things that must be done and/or they’re bugged.
But since you’re looking for Official servers, the age is in days active with no wipe and accurate. Most were opened at the same time.
So it won’t have an impact on determining which servers are busy.
The fact is that every single Official server has some population on it. Whether it be 1 person who logs in once per week to refresh their base or hundreds of despawned bodies. If not thousands.
To my knowledge there are no lists and if there are it would be personal ones.
Unfortunately the only way to find out is to make a character on servers that you see low population on and explore it. Do it during primetime for gaming.
I would have suggested websites such as Battlemetrics but I believe the servers are no longer being updated on there.