excel sheet work with functions/formulas

Re attached excel sheet: -Use the tab "Tax Rates" for calculating the total taxes paid in columns S-T-U & V-W-X-YZ starting in rows 21-76 on both tabs "RIF Max as couple" & "RIF Min as Couple Status Quo". Currently it uses the "Tax Table" in rows 11-17 for the calculations. For the tax calculation as an enhancement from the existing calculations I would like to add an IF so that if the age in column D-E is less that 65 that the calculation is based on the "BC under 65" table in the "Tax Rates" tab and if 65 or over the calculation used the "BC 65 and older" table.