You should apply for SSDI as soon as it starts effecting your ability to work. The application process often includes SS Administration requesting testing. Your delay in applying has drastically reduced the monthly benefit amount you will receive. They average your income over the past years to come to a benefit amount. The years you have not worked will reduce that amount.

So, get the application started. You can start it online at https://secure.ssa.gov/iClaim/dib