Single mother of disabled child

I am a single mother who struggles. I am unclear why children under the age of 18 or in High School do not apply for the one time payment.

My son is 17. I am sorry but this does not seem like a fair thing. I just spent $300 on a ramp???? It seems like the ones who are busting thier butts to get through are the ones left out. Not fair. We have major medical necessities- other recipients do not have the major medical needs that others with major disabilities have...