DS will be five in March. Perhaps we missed that boat.
You can absolutely submit an SSI claim for your child at almost any age (under 18, over 18 and it gets a little trickier depending on their disability). I submitted Kearnan's just after his 5th birthday and he was approved almost immediately. I am considering applying for my youngest who recently turned 7 and was diagnosed bipolar. My understanding is that he will not qualify for income or if he does it will split what his brother receives, but by qualifying he will then qualify for medicaid which will help with his prescriptions and therapies. I do not know anything about regional centers though, I think that must vary by area. SSI is different from state to state. As far as I know a child/family must qualify both by disability as well as by income level however in some areas a child can be denied based on income, qualify based on disability and therefore qualify for certain medical needs benefits like medicaid. It is certainly worth pursuing.