almost all of this is trivial to say "they just evolved that way". Species of all kinds of intellectual levels have evolved, with different lifespans and sexual maturity. For the most part you can get away with saying "they are just that way". The only real world building issue I could fight over is the intellect levels vs tools.
15 year olds are sometimes rather emotionally immature, but they aren't idiots. a 15 year old can make far better then 'crude tools' himself, I've seen some really skilled 15 year olds. Once taught how to do something they can do it pretty well, and it only takes one smart 15 year old to figure out how to do something. Plus, if they live around smarter humans they just have to be taught by smarter people how to make better tools and then remember. In short, a race of 15 year olds should have more then 'crude tools'.
Bonobos have crude tools, bonobos raised by humans can cook meals over fire or on a stove, work light switches and even touchpads, and generally interact with the tools we provide them pretty well. If you make a species much smarter then bonobos your have to put more thought into how their intellect vs their tools interact to avoid the, all to common in fantasy, mistake of making 'primative' tribes that are too smart to continue living so primitively.
In particular if your orcs are near humans or other intelligent races they will be getting tech and ideas from those. Even if they are not smart enough to create they can be smart enough to USE it.
You could have them be smart and their lack of tools is simply due to lack of resources to build or make them, the same way that third world countries still rely on 'primitive' hunting techniques for food because, even though anyone can tell them how to build advanced guns, they still don't have resources to make, craft, and maintain a large number of them. This would be the easiest approach to explain the tool vs intellect dependency. However, then you need to explain why they have limited resources, and why they don't just trade with, or raid, nearby humans for their goods.
Alternatively you can make them actually less intelligent, with actual cognitive limitations preventing understanding the use of advanced tools. However, if you do that your want to put some real thought into how to make culture and technology equivalent to their actual intellect. In particular avoid trying to say "their just like us humans, but stupid". They won't be able to have the same sort of conversations and philosophies as humans if they are less intelligent, this would also effect culture and beliefs and many things. Their cognitive difference will result in a very different species then humans. This is worth it's own separate world building question actually, development of culture and technology for a cognitively limited sapient species.
IF you want to go with hard science approach I could nitpick on some other minor details, though I think they are all small enough that you could be forgiven for taking poetic license on them. The rest is major nitpick and your welcome to skip it!!!!
First, sexual and mental maturity are awfully far away. Sexual and mental maturity should be closer together. Humans are much closer when it comes to our mental and sexual maturity. We may consider 20 to be be 'adult' now of days, but we have developed most of our mental abilities long before that, we have room to learn more, were always learning, but our cognitive abilities are mostly complete at a much closer age. Besides, if we reach sexual maturity around 10 then we would have to reach mental maturity at 30 to keep with your ratio.
Related to issue of sexual maturity: For orcs to evolve intellect they would need to be heavily K selected species. K select species don't tend to reach maturity at such a young age anyways. Look at most other primates. Bonobo reach sexual maturity at age 9, and their err...sexual lifestyle would encourage quick sexual maturity. Chimps reach sexual maturity after humans. It's the same for nearly all 'intelligent' animals, they have long childhoods, and these creatures are all less intelligent, and smaller, then your orcs.
There are many reasons for this, the short version is that intelligence means less instincts, you need to learn how to use your intellect to survive. You won't survive on your own until you have learned, and so you need parental investment much longer which generally means delayed sexual maturity. This is only one part of it, all K select species have delayed sexual maturity and sapient practically necessitates K select strategy.
Your orcs are also much larger then the other intelligent animals. The larger a species is the longer it takes to reach sexual maturity in general.
In short for sexual maturity to be realistic I would push it back to closer to us, 8-10 lets say. I would also say that the males at least must wait much longer before able to compete sexually for females. Rather or not the females begin to reproduce as soon as they reach maturity depends on your society, but most likely they would if your assuming a less intelligent species then humans. Lower intellect means their less likely to have complex cultures which would discourage female from reproducing 'too soon'.
If you really want reproduction at a young age It could be done, but only with selective pressures. Maybe carrying a child to term is difficult enough that they start reproducing early to have more years of trying to conceive, with parents helping to care for grandchildren when their children are not yet ready to care for the child. However, your need either very high infant mortality (but very low death in childbirth), or simple difficulty with conception or carrying to term to make this work; along with grandparent investment being more common then most animals. Of course grandparent investment would in turn place evolutionary pressures to encourage more intellect to evolve in the orcs technically; but maybe they are evolving to greater intellect and just haven't gotten to our level yet.
However, these are both nitpicks from someone who really likes considering evolution. I'm pretty sure you could ignore most of this and have few, if any, complain. I would not scream in outrage if you had your orcs as you described with little further justification. So long as you don't also make them always chaotic evil that is :)
