Never gave much thought to this, mainly because I admittedly haven't read much of Johns' GL run, either Pre or Post Flashpoint. But I understand the issue - its the same one the Batman franchise faced after the New 52 with the Robins. 4 GL's in 5 years!
In GL's case I suppose it works a little better than it does with the Robins because they aren't kid sidekicks but members of an intergalactic police force who could be recruited fairly rapidly one after the other.
Hal could have been recruited 6 years ago (around the same time Superman and Batman started out). Guy and John could have been recruited 5 years ago. And Kyle around 2 years ago. It's far from ideal, but makes a hell of a lot more sense that Damien being 11 while Bruce became Batman only 6 years ago