As stated here, if you are 65yo, or over,you get the full rate, provided 65yo was your SSA retirement age. If you are/were younger than your SSA retirement age, you would get the full rate from SSDI. If you are over your SSA retirement age you won't get SSDI, due to the fact that once you achieve retirement age your funds come from the SSA retirement acct. If you were disabled before retirement age and you can win, an EED for SSDI, you "might" be able to get a retro check.
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