I think some of them are pretty good! My favorites are the Ciaphas Cain series and Eisenhorn series. Gaunts Ghosts is also good if you want a more serious military read. If you want non-humans, check out “The Infinite and the Divine”. It’s about necrons (highly advanced ancient space robots/terminators).

He meant that you’d have to start fathering children at 14 so that you could have 4 of them under 18 by the time you turn 18, which would be the age for military service.