You may be entitled to the Special Guaranty computation at any age if you are eligible for a disability freeze under SS Act rules. It means that you are considered totally and permanently disabled for all employment, the disability is expected to last at least 12 months, and you have the required Quarters of Coverage under SS Act rules.
The disability freeze is determined based on the medical evidence submitted for you and based on your earnings record. All of your earnings from railroad and social security-covered employment are used as if they were earnings under the SS Act.
The disability freeze is further explained in booklet RB-1D, "Employee Disability Benefits".
Your Special Guaranty benefit based on disability is not reduced for age.