I think the wording is fine. While technically you are correct, most people will not interpret "three of the oldest" to mean that they could be 2 seconds old as long as there is another star which is 1 second old.
It's a common journalist tactic that allows for painting a misleading picture. E.g., I read a story earlier today about "State X among the worst states for Y". The state could be #2 out of 50, and they'd be "among the worst 49". The state could be #49 out of 50, and they'd be "among the worst 2".
Same thing here. Three of the oldest stars? It means literally nothing but it paints a picture.
There could be a quadrillion stars of a similar age and the statement could still be true.