a child will grow up to have a score of Dx:3, but as a child it is at -4, so they have a DX-1. A magus comes along and, seeing the child's potential, uses ritual magic to raise the score from -1 to 0. (presumably from something more than the kindness of their heart) if the CrCo spell is only strong enough to raise the score to 0, what happens when the child gets older? Does the magus' foresight pay off and they grow to DX:4, or does the spell fade when the child reaches an age where their dx is already at 0 or above?
All characters have a score. Child characters have a modifier based on age. Their score is their score, while their modifier changes as the character grows. So to my thinking Dex 3 is the score, and raising it by a magus would require a ritual that would raise the score from Dex 3 to Dex 4.
I'd vote going with the higher of the two scores, with one being Dex-1 +1 (capped at max of zero due to magnitude of spell), and the other being Dex +1 or higher due to growth. So basically you grow out of the spell.
Just like if you cast two Gift of the Bear's Fortitude, only the better of the two count. I understand this is comparing a momentary ritual to an ongoing effect, and I am choosing to ignore that difference to arrive at a simple, workable answer.
I agree with Mr Link in this case. I could envision Magic working either way but it stays cleaner if your Creo buffing rituals targeted/improved the pre age modified score.
Mind you if it worked the other way I would so love to have a wizard aquire infants raise their intellegence to +5 just so they could see in 15 years what a person with +9 intelligrn id like.
That's some kind of scary smart baby...
Scientifically, their brains would most likely-
Wait, this is Ars Magica.
... Maybe they'd speak Adamic?
The 'great intelligence x2' character has a modified intelligence of 0 before they make it past infant. That's ... kinda scary, actually.
Especially when you adjust the story slightly then pity the parents of the character with mythic strength. Because having a toddler with a positive strength score would be so awesome?
Wait, what? Great Int x2= 5. The Core book has ages 6 as the lowest age, which has the -4 modifier. Apprentices adds ages before 6, which have larger modifiers. -10 is for ages <1 year, and decreases by 1 for each year from starting at age 1 and continuing until Age 6.
Wait I forgot that. So Baby with +5 characteristic equals 6 year old with a +11 equals adult with +15 
Quick can anyone get me a thousand pawns or so of Creo Vis and a Gifted baby there is something I just gotta try.
Well, the characteristic doesn't change as the person ages. So... No. 
I should have specified.
Infant as in 'infantia', not infant as in 'screaming baby'
Personally I'd never run stories for characters younger than 2, because regardless of what the rules say a person of that age is more akin to an object than a self-directing, active participant.
Having a modified intelligence of -1 at the age of 4 is still freakish. 4-year-olds being able to outsmart grown adults is just wrong.
Having a modified strength of -1 at age 4 is definitely scarier, though. Especially for the parents.
"4-year-olds being able to outsmart grown adults is just wrong". Have you never babysat? Children are sneaky, smart, and evil......
There is also the Deft Characteristic child virtue that halves the Age Modifier. So a 4yr old could theoretically have a Characteristic as high as +2. Freakish yes but a bit more believable if your talking Quickness, Presence, or Stamina instead of Str or Int. Scary either way though.
Heck, even those are scary in their own right, though probably Stamina is the least so. Think about what that Presence score would constitute: that basically throws everything about "not taking kids seriously simply because they're kids" right out the window, since that kid has just as much or more charisma than most adults. From the outsider's perspective, this would basically be a child getting just as much respect (and I mean actual respect, not "aw the little kid is so cute" attention) from patrons as they're giving to their fellow adults. That's almost mind-boggling, given the strong natural tendency to treat children as foolish and/or inferior. And Quickness? I mean, kids can dash around a lot, but most toddlers certainly couldn't escape from a dedicated pursuing adult for even a few seconds. This kid? He's running and dodging and bobbing and weaving and you never thought you could see such small legs outrunning an adult. It's not quite as mind-boggling, but that'd be really weird to see in practice.
I'm a parent. 
We're now at discussing a characteristic score of 0 at actual birth.
Which honestly just tells me that the system doesn't really work for very young children. I'm happy sticking with my minimum PC age of 5.
I don't find it all that broken.
Someone who has any characteristic at +5 has spent 6 of the 7 characteristic points and 2 virtue points on it. Take a child who is 5, who has done all this, and adds Deft Characteristic and yes, you can get to a 0 at birth, but what can someone that age do? He might figure out walking by 9 months...
Even a 5 year old who has an effective score of 3 with all of the above virtue and characteristic points spent has some challenges. That freaky strength? Well, he can't apply it well, because his Dexterity is lousy. Intelligence, he's a prodigy and he's bored by all the children around him, and doesn't fit in well with adults, and so on. The really nimble (Dex 5) child can land his punch, but his strength is -5 (assumes Str 0) which completely offsets any bonus damage.
The presence score Akriloth mentioned isn't really a problem, because you are ignoring the fact that it is still a kid. This would be akin to how a lot of times people ignore that the Gift starts off from a place of mistrust AND has a -3 to the die roll. Context matters in story driven games.
A lot of this conversation seems to be ignoring the part about growing into Virtues/Flaws, not just growing out of penalties to Characteristics. The rules do not say that just because a child will grow into Int +5 that they must begin life with Great Intelligence x2. Those Virtues could be picked up later in life.
As for brilliant children, consider the anecdotes of Gauss as a child.
Agreed from a real world stand point. But from a narrative stand point?
High stamina that's easy. Kids as healthy as a horse doesn't get sick, falls out of trees and doesn't get hurt, boundless energy. So other then the sick thing your average four year old.
High Presence. Don't think about it as respect. Think gets what they want through shear force of will. Think I'm cute and I know it. Think Tantrums from heck. Basically a spoiled princess.
High Quickness. Weird to see yes, but you won't necessarily see it. They will be the kid who is never where you left them and get into trouble the second you look away. Turn to put a basket down and they are halfway across the field. They wander into the road and never seam to get hit by the carts. Slip through a crowd while your stuck pushing through.
Also don't forget they are going to have seriously low Abilities. It's all raw un-directed natural talent.