I am 35 years old and have been on SS disability for 8.5 years now. I applied the first time when I was around 27 and was approved in only 6 months due to some bad health problems.
I had no help really and honestly didn't know what I was doing. Now all these years later I realize that I should of filed my onset date for a year earlier than I did. Based on my earnings at the time I would have gotten a bigger disability check each month right now and I would probably have gotten some back pay.

I believe I actually became disabled when I was 25 years old but I didn't file until I turned 27 and I had listed 27 as when I became disabled.

I realize its been many years but would there be anyway to go back and try to get it adjusted or amended now? I'm sure there is probably a statue of limitations on amending your disability claims but I'm not sure where to find it.

I made more money when I was 21 years old then I did when I turned 22 and if social security went back 5 years to determine my disability amount then my current check would be bigger if I would of filed my onset date starting at age 25 instead of age 27.

All of your income is considered when receiving federal SS benefits. So it is your total earnings they take into consideration not a basse period. So it down't matter what you were making when you applied. State disability programs use a base period but not the federal government.
As for getting your onset date readjusted that's a tough one. Obviously, you would need evidence to prove the earlier onset date. It wouldn't change your monthly rate but it would change your back pay benefits. If you were earning more than about $680 a month 8 eight years ago (during the year before your applied for onset date) you wouldn't be eligible anyway.
Call Social Security and ask them. I would be curious as to what they say.

There is a four year limit on reopening SSDI decisions, either favorable or unfavorable, if the decision was not still under appeal. So the decision that was made 8.5 years ago cannot be reopened under the rules of administrative finality.

Onset date can change the monthly benefit amount (generally higher but can be lower) because there are fewer months in the PIA computation period. It may or may not change the months of retroactivity depending upon the date of filing since there can only be 12 months of retroactive payment from date of filing. Also, you may not have sufficient medical evidence dated 10 years ago to even prove that you were disabled then.

Bottom line: you are too late.