Two personal communications have added important modifications to the idea expressed in the previous post.
The first comes from Rob Jones and concerns the analogy with the Napoleonic Imperial Guard. Rob points out that the career path proposed for the iuniores and seniores would not work for the imperial guard (promotion from the Young to the Middle and then Old Guards, before taking up a position as NCO or higher in the junior regiments). To be honest I am not sure how exactly I meant the analogy to be taken (it being some time since I wrote the original foot-note to my CHW chapter) but if I meant it that exactly then I was wrong, and certainly it can be read as implying that. It's possible that the analogy could stand in a 'weak' sense, in that the words 'Young' and 'Old' were simple references to the respective age and experience of the troops in the units concerned. Rob suggests that a better career path analogy would be the formation of 2nd battalions from cadres in the 1st battalions of British regiments (and others).
The second addendum comes courtesy of Paul Browne, who says that M.J. Nicasie, Twilight of Empire: The Roman Army from the Reign of Diocletian to the Battle of Adrianople, pp.29 ff., is of use to the enquiry. I should check that out - indeed I should have checked it out before now!
Thanks to both!