Hans Müller retired in 2009 and received a pension of 12.000 Euro in 2023. With a taxable portion of 58%, 6.960 Euro are taxable. His allowance is 5.040 Euro. As long as his income is below the basic allowance of 11.784 Euro (2024), he does not have to submit a tax return.
If Mr Müller only retires in 2024 and receives an annual pension of 15.000 Euro, 12.450 Euro would be taxable (83%). Since he exceeds the basic allowance, he would have to submit a tax return.