In my opinion, most adults over 30. But that's just my opinion. You can be tested for IGF-1 levels, or serum levels if you want to determine your needs precisely, and have your doctor decide if you are sufficiently "hgh deficient" to a degree they feel you should begin injections or supplementation. I've never heard of the spray hurting anyone regardless, and again, my opinion is anyone over 30 could use it, but again, that's just what I've heard and from my own limited knowledge, and my own opinion, not a doctor's.
I’m middle age and have been taking it for the anti-aging and age reversing effects, including immune system enhancement, sex drive, bone & muscle, and decrease of weight and wrinkles, for years. I've only recently had my blood tested. Personally, if I were 30 I would be taking it to help preserve my youth, instead of trying to get it back and repair the damage already done. If I were a body builder I’d even take it in my 20’s - but that's personal choice.