SSDI is for disabled workers and their minor children and their disabled adult children. Your 9 year old qualifies because he is your minor child. The issue of disability does not apply to him for SSDI. If he is still disabled after he turns age 18 and is unable to perform SGA (work), he might qualify on your record.
There is the low income program, SSI, Supplemental Security Income, that pays money to disabled children and disabled adults and people over age 65. What they all have in common is that they have limited amounts of income and limited amounts of resources.
But if your intent is to have him declared disabled at age 9 so that he can still be declared disabled at age 18, it won't work. The criteria for children is different than the criteria for adults. All SSI children get redetermined under adult rules when they turn 18. It is totally possible for a 17 year old to qualify for ADHD, but not qualify for ADHD as soon as the child turns 18.
And you were obviously able to perform enough work to be insured for SSDI until your condition got worse, so it is doubtful that your parents could have done anything for you to be declared disabled earlier. You may have had problems, but you managed to work anyway.
You can file a claim for your child for SSI now. The amount SSI pays can be as low as $1 or over $750. All income the child receives, like benefits paid on your record, child support, TANF etc. is used in computing how much SSI can be paid to your child. Also, SSI will look at your total income sources as well as the income of your spouse in order to determine if the family has low income. SSI will also look at all of the child's assets, your assets and your spouse's assets (bank accounts, investments, real estate, multiple vehicles, etc.)
Your child's medical condition may or may not be severe enough for him to even qualify for SSI disability and your family income and resources may or may not be low enough for him to qualify. SSI is a pretty invasive program into your personal life. You have to provide private details and evidence about income, resources and living arrangements of the child. SSI can change the amount of the payment every month, if income changes every month. Many children end up overpaid on SSI because of underreported changes. If you do decide to file for SSI for him, be sure to read everything you can so that you understand it so that your child doesn't turn 18 owing money back to the government.
The best way to get a thorough answer is to file a claim and provide all the information. SSA should first determine if the income is low enough before making a decision about whether or not the child is disabled enough. The computations are complicated as is the disability decision.
With SSI, no payment is made before the month of filing. So if you choose not to apply until some time later, the earlier months won't even be considered.