it's never too late to file your taxes, but it can be too late to get a refund (but that time is usually three years after the April 15 deadline.)
If you file late without an extension you may be subject to a late filing penalty, which I have not seen assessed if you are getting a refund. If you don't pay on time, they you will be charged interest and perhaps a failure to pay penalty.
People who didn't file 10+ yrs ago even file paper returns because the IRS will eventually make up a return for them based on w2's and the like. So the IRS assess taxes based on no deductions and therefore folks can frequently get tax owed reduced (but not a refund).