There is no single right age, but there are hard deadlines: the juvenile procedures that redirect hip development, JPS and DPO, only work in young growing dogs, JPS typically before about five months old, which is earlier than most owners have even heard the diagnosis. After growth, the calendar matters less: FHO and total hip replacement work at any adult age, decided by pain and function rather than birthdays.
The practical takeaway for a puppy diagnosed early: see a specialist promptly, not because surgery is inevitable but because some options expire. A screening conversation at four months costs a consult fee; missing the JPS window costs an option.
For adult dogs, the age question inverts: not is my dog too young, but is waiting costing comfortable years. A three year old failing on management has decades to bank from a definitive fix; drifting to age seven on escalating medication spends those years without buying anything.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.