Small Business Tax Extenders Act of 2012 - Amends the Internal Revenue Code to extend through 2012: (1) the 100% exclusion from gross income of gain from the sale or exchange of certain small business stock; (2) the five-year carryback of the general business tax credits of eligible small businesses; (3) the offset against the alternative minimum tax (AMT) of general business tax credits; (4) the reduction (from seven to five years) in the recognition period for the built-in gains of S corporations; (5) the increased expensing allowance for depreciable business assets, including computer software; (6) the special tax rule for long-term contract accounting; (7) the increased tax deduction for small business start-up expenditures; and (8) the tax deduction for health insurance premiums in computing self-employment taxable income.