Only recently in aus did an adult diagnosis get support from the government PBS scheme. And even then it's because they're letting retroactive childhood diagnoses be considered for the subsidised medication options.

In some countries it's almost impossible (I don't have any direct experience but I've heard tale that in e.g. aus/nz, east asia, south asia, etc people give up before getting diagnosed often), and my understanding is that in Europe it's virtually unheard of to have an adult diagnosis.

In the US, at least, it ranges from difficult to straightforward, but that's not true everywhere