If your military is growing prolong is a wonderful thing. New commands, an increased need to career NCOs and Officers, and the ability for dedicated individuals to earn promotions and move up.
The SKM got prolong at the perfect time. Just as they were starting to drastically build up their fleet was the same time that prolong was universally available to the population.
Now, you have the ability to set 100 year plans for your officers. Have a promising JG Captain go dirt-side for 2 years as a training officer before getting his first cruiser, and then spend another 3-4 years as a staff officer for an Admiral before getting a BC. They could have officers take 5, 10, 15+ years out of active, space duty learning other aspects of the service and refining their skills without wasting a large percent of that officer's career. And, at the end of the day, an Admiral who has spent time as a training instructor, staff officer, weapons development liaison, and logistics command for 10-15 years will give them an incredibly broad view as a flag officer.
The SLN, on the other hand, got prolong when their fleet was well past any real expansion. Thus, you had connected officers continue to get promotions while serving the same function. Officers without connections, on the other hand, would top out as JG captains of destroyers or perma-XOs to better connected (and much younger) captains of cruisers. All prolong did for the SLN was worsen the nepotism - because effectively quadrupling ones effective time of service (say, 40ish years to 150+) without a corresponding increase in the fleet brought merit advance to a stop.
How this works with Manticore's population is unknown. For example, who is to say that around 1950 Elizabeth and Justin won't decide to have another child? After spending a couple decades raising children, and then 7 decades without them, maybe couples start deciding to have a second 'batch' of children. They certainly have the time (in life span) to effectively raise 2-3 'batches' of children with multiple decades off in between. The point is, we don't know - because the first generation to receive prolong is still around and the full effects have not worked themselves out yet.
Personally, I see a lot of people (with the economic means) being like Honor's parents. Say, you wait until your 50s to have your first 2 children. By your 80s, empty nest. I could see a strong pull around the 100s-110s to have another child or two for many who genuinely enjoyed raising kids. Unlike our world, where by the time the kids have grown and left and people start missing having kids to raise they are unable to have more, in HV they'll have another century plus to give it another go!
I think that birth rates (births per year) will actually decrease from what it was pre-prolong, but birth rates (births per parent) will increase as many families have a second (or even third) batch of kids. Over time, that will lead to a much larger population just due to longevity. And, a much more intelligent and useful population.
