36 yrs old & have hip osteoarthritis from injury long ago. 2nd surgery last year, pt & injection. Still pain. When is it ok to have hip replacement?

Last Resort. You are never "too young" to have a total hip if your hip is irrevocably worn out. I'd be interested in what the previous surgeries were. The total hip replaces the hip joint. There are a lot of causes of "hip pain" that don't come from the hip joint. In those cases a total hip wouldn't help. If the hip joint is totally worn out, then a total hip would help.
It is up to you. I believe when you no longer can live with pain. Having said that, you should understand this surgery may have to done again because your age, what kind of work you and many other condition. Some of those surgery could done were resurface the bone i will recommend that if possible.