[anonymous]

Often there is a genetic component, so age doesn’t matter. It maybe best to ask your vet for a referral to a specialist (internal medicine) if you have not had positive results from treatment so far.
At 18 months your dog is a mature adult.

PS: It may be that your dog is “reluctant to pee” because it is painful to urinate.
Were x-rays/ultrasounds done? Bladder stones ruled out? Talk to your vet.