Rough numbers - somebody will be here to correct me but you’ll get the idea:
Join as a SH FO. After 5 years, you will probably be reasonably senior on the SH FO list, and so achieve a pretty good degree of roster control in terms of getting the days off and types of trips which you like. After 7-8 years, you would probably have very good control. After 10 years, you’d be more or less creating the exact roster you like. This is because gradually those F/Os senior to you move on to other fleets or seats, and everybody joining the fleet is junior to you. Standard disclaimer is that the peaks and troughs mean that this move up the list may take the form of periods of stagnation of a couple of years, followed by rapid movement during periods of high recruitment.
S/H command theoretically possible after a couple of years at the moment, however for the next couple of years it could stay that way or be more like 5-7 years or even more. Impossible to predict. However, you bid for what you want and if your number comes up then great. That said, it is unlikely you will feel senior enough to have good roster control until maybe 20 years or even more, and therefore whilst the prospect of an early command may seem appealing, it will always come with less than ideal rosters for a long period of time.
Moves to L/H FO positions possible/likely after initial engagement freeze of 5 years, although in periods of stagnation there may be slightly longer waits - luck of the draw. However the same principle applies - the earlier you move the longer you will be junior for on your L/H fleet. Lots of people remain as ‘senior’ L/H FOs because after 15-20 years they gain lots of control over their lives. Movement up the list therefore is pretty slow.
L/H captains maybe 20 years minimum to be more or less the most junior on the fleet, and remain junior for a number of years. There’s a lot of captains who joined in their early 20s maybe thirty years ago, so they have thirty years of seniority and could theoretically stay for another decade. So really, to be a L/H captain with decent roster control, you will need to have served 30 years plus.
So to answer your question…yes, you could maybe join at 30 and remain as a 320 FO, giving you good roster control by the time you’re in your late 30s. You could hang around there enjoying life until mid forties, at which point a move to L/H FO would give a reasonable degree of control. By the time you’re in your mid to late 50s, you might start considering a long haul command and in your last five years (if you stay to 65) you would have fairly decent rosters. This is why you will hear people say that ‘you only need to be junior once’.
Moving through fleets or seats at the earliest opportunity will mean that you will spend virtually all of your career as a junior bidder, until your last few years when you are finally senior on any fleet you choose.
Lots of health warnings on all of the above…situations may change a lot in the course of the time scales being discussed, and so that’s just a bit of a ‘snapshot’ as to how things stand now.
Caveat emptor!