... From the uvby photometry and proper motions for about 5500 nearby F stars we have found the following: (i) F stars , taken in narrow ranges of metallicity , show at [fe/H] 0 rather distinct cut-off in their distribution along the Main Sequence (MS) at the blue side, which is suggested to be an indication for the MS turn-off in stellar groups of fixed metallicity ; (ii) the corresponding turn-off age from theoretical isohrones strongly correlates with the ...