Is failure to file a tax return reason enough to subject one to criminal charges?

I have many clients who have not filed a tax return in 10 years or more.  These clients do not need to worry about going to jail unless he or she set out to deliberately evade taxes and cheat the government.  One goes to jail for fraud and tax evasion, not simply owing tax.  Assuming that is not the case, I can help my clients get back in the system.  First, they will have to file their returns from at least 2010 forward.  Generally, the IRS will not require one to go further than six years back, plus the current year (still 2016).  I can obtain their income records for those years to help determine income, but, of course, they will need to provide any expenses they want considered as deductions.  After the IRS adds on the interest and penalties, we will know how much is owed and work out an Offer in Compromise or Installment Agreement to retire the taxes.  To do that I will need to get a complete financial picture using forms I provide.