Uh, what?
IMO, Perdo Corpus on a child would probably regress their age. Creo Corpus on a child will advance the age to maturity. Perdo Corpus on an adult will age them. Creo Corpus for adding or subtracting age fails.
Fundamentally age can never be regressed. However if you aren't spending vis then any CrCo aging should be temporary. I suppose with duration moon it could be used to remove youth penalties to int for an apprentice...
Agreed. I'm not even sure I even accept that there are implicit guidelines, and would say someone is inferring, rather than the authors implying that something is possible. Prior to guidelines to move wood, this could be rather easily accomplished with ReTe and a herbam casting requisite. While the casting requisite does make it a more difficult spell for some, it's still only level 5 or level 10 spells.
The guideline is to bring to maturity over the course of a period of time. It's up to the saga to determine whether this is of ritual or power or not. I would lean towards no, because the person still needs to learn, and the spells that follow these guidelines don't handle any of that.
Vis, fundamentally, is money, that's all it is in Ars. So, suggestions that vis be used as the yardstick to determine what is and is not possible in saga are probably faulty. Yes, magi can make things with vis, powerful things, but they take time, and one can get into a make or buy analysis: pay vis and time or pay 3x vis and spend time doing something else. The guidelines don't mention the need for vis, but one can always go back to The Central Rule and decide that these spells are too powerful to be mere formulaic spells and make them rituals. I'm not sure that it benefits anything though, because the individuals affected by these spells still have a lot to learn, that they missed out on.
Aging an apprentice, temporarily, seems like it should work, but I find it odd that CrCo will age them, and remove then penalties to Int, when it seems that CrMe is a more fitting TeFo combination... I know what the aging chart suggests, but that also presumes natural advancement...
On the other claw, as argued elsewhere, int is a fundamental characteristic, not an issue of development, which is why you can't use a ritual to raise someone's int to 5 when they are age 5 then watch them grow up to Int:9. If you could age their mind with CrMe that would suggest to could add xp to abilities as well...
Well, their Int is already +5, they have a penalty based on age. So, I'll grant, that the RAW seems to suggest that aging someone supernaturally does allow for removal of the penalty, but that's because the RAW presumes natural aging.
This is one of those cases where I think an implicit (SWIDT?) guideline that ages the child mentally, removing the mental characteristic penalties should be necessary to differentiate from the physical characteristics. I like it because it creates a bit of a Freaky Friday effect, older person in a young body or a young person in an older body (also think Big!).
Children learn in weeks what it takes months or years for those already set in their ways to learn.
As is often very noticeable in regards to modern technology. 
And children by default accepts the womb, OR something similar, as safe and comforting, meaning that the extreme closeness would most likely cause children born to a woman with the gift to feel very "at home" with it, even beyond just "getting used to it", as it will be a very strongly noticeable part of their "first impression".
You may very well get the occasional "opposite reaction" that gets somewhere close to what you describe, but iĀ“m fairly sure it would not be the norm.
IĀ“ll have to say that effectively they do, because itĀ“s part of their mother. And to at least some extent that is likely to carry over towards others with the gift.
This is a completely different thing, not even slightly comparable. The mother is already adult and set in her ways, and i very much doubt that the gift of the child will be fully noticeable from the start, or possibly at all during the pregnancy.
Sure, but they have things like learning to walk, fine muscle control to name but a few, and there is growing evidence that picking up on technology comes at the expense of something else.
Finally, even without the Gift, teenagers will grow to resent their Gifted parent, so...