You have to maintain a spread of ages and experience across a Branch or Trade. You need sufficient people who are young enough to maintain a structure and who have the potential to achieve the highest rank possible and have full careers. If the pyramid (and it is a pyramid) gets to be the wrong shape (a 'bulge' around the middle just like mature ME crew when they haven't moved for a few years) then you get promotion slowing / stopping and it all goes wonky at that point. Also, if you switch off a training stream completely it is very difficult indeed to start it up again.
So. Some out of a mix of ranks and seniority, some more in at the bottom to keep things moving.