I think the best age for GH is the one where you bring in a generous salary, have a house and a strong diversified portfolio and you still have a nice surplus to spend on frivolous things like GH.
I think the most effective and cost-wise thing you could do with GH would be to run it in tandem with insulin to break through to the next stage. Outside of that, it's just really, really, really nice to have. You can get big or you can get shredded without GH, but if you have GH then you'll get bigger and more shredded with ease.