Not to be all Junior-Moddy, but I don’t know how a factual answer can be given for this question, so I’ll just give an IMHO answer, and you can ignore it if you want.
First, to echo what others have said, at no time in history (that we no of) was 30 the maximum age. So we’re not really talking about returning to an historical context. We’ll have to take your scenario on its own terms, not comparing it to ancient times.
Assuming a maximum age of 30 and a mandatory breeding age of 15, I think we’d be a lot *less * advanced than we are today. For one thing, we’d have no grandparents to watch over the kids, freeing up healthy people to do heavy labor. We could change our society to one of kibbutzes, where a few people watch over all the children, but I have to wonder how this affects the drive to reproduce.
I don’t see how this assumption is valid. We’re not talking about fundamentally changing brain structure.
Losing our elders means we lose a lot of knowledge. Of course, we could compulsively database all knowledge, but if you’re dying at 30, who has time to read it all? (Already, I assume I may live to 90, but I have no hope of gaining all existing knowledge of arts, literature, computers, engineering, science, mathematics and laundry washing. I learn what I can about what I can and trust that others will keep my car running for me.) Since so much of our development is based on first comprehending and then adding to what others have done, the rate of development would be greatly slowed.
Think of it this way. An engineer currently has about 25 years to learn all she needs to know about the basics of engineering (elementary school, high school, college, grad degree). There’s a period of a few years where she’s still basically repeating what’s gone before, perhaps refining it, but essentially just getting good hands-on knowledge of what was formerly book work. Only when she’s 30 or so is she really becoming capable of groundbreaking, original work - the work that her successors will learn about in their own schooling. Obviously, there are prodigies and exceptions, and there always will be, but I think your scenario will make it much harder for advanced work to occurr in any field. You might still ahve an Einstien, but far less people inventing useful things like toaster ovens and flushing toilets. (OK, I can’t find a bio of Albert Giblin, maybe he was under 30.)
Now, a lot of the educational time is spent learning about stuff seemingly unrelated to your eventual field. Our hypothetical engineer had to learn not only math, physics, and science course, but history, languages, spelling, music, art, P.E., etc. Perhaps we could cut that training time in half if we didn’t make her learn the liberal arts stuff. The problem with that is that there’s just a limit to how fast most of our brains develop. No matter what kind of background I give a ten year old, he’s simply incapable of learning Ph.D. level physics. His brain hasn’t formed enough neural connections yet. But let’s assume we can do it. We can teach this girl everything she needs to know about engineering by age 14. (This of course assumes that we assign everyone their eventual career in early childhood. We don’t have time to let people figure out what they like or what they’re good at. Early apptitude tests and no career changes.)
Oh, well, now she has to get pregnant, in order to reproduce at 15. (And we are going to give her a little time with her newborn before shipping it off to the kibbutz, right? 6 months for breastfeeding, perhaps?) So now she’s rounding on 16. Four or five more years to get hands-on experience and really let all that knowledge sink in and let her make a few mistakes. So now she’s 20. She can finally start doing some really orginal work. Wait a minute, who’s teaching the kids now? Is it only people who have absolutely no hands-on experience? That may work for little ones, but once you get to advanced work, you really learn better from people who have worked in the field. Except you need those people actually working in the field.
Finally, of course, is the question of whether or not people will be willing to devote their entire, brief life to work and reproduction. In developed countries, we currently enjoy a lengthy retirement period before expected death. Many people use this time to be with loved ones, travel, learn and basically enjoy life. If you allow people a retirement period, you’re seriously diluting your workforce. If you don’t allow them a retirement, morale (and therefore production) could be expected to go way down. (This is assuming health and stamina of current 30 year olds before sudden death. If you people are getting “old and frail” near 25, see above and multiply the problems.) In lesser developed countries, there may not be a formal “retirement,” but less is acomplished over the entire lifetime. And the country is “less developed” as a result.
So overall, I just don’t see it working too well.
(who’s obviously spent way too much time philosophizing in coffee houses.)