8gb of ram will mean that it runs more programs simultaneously, or more complex programs without running out of steam. While it won't be required now for anything you have mentioned, if he genuinely uses it for 5 years it's hard to predict what he will be doing with it in 5 years time. To be clear, 4GB will be fine for his uses now with what you have described.
So my advice would be, if you can find a refurbished machine with a larger ssd drive and more ram than a new base model, go for it, as it will last just as long (if not longer) and be better value for money.
I just asked him everything he would use it for, this is what he told me
-School (Powerpoints, Word docs, research, taking notes, web apps ect.)
-Creative work (A "definitely legit" copy of Photoshop CC for photo editing, he might also lightly use "legit" copy's of After Effects, and Flash to make animated videos, he says he might also edit clips with iMovie and the odd song with Garage Band)
-Web (Social media, web apps/flash games, forums, youtube, reading blogs/blogging)
-Light gaming (The odd small indie game/roguelikes)
4gb of ram should be able to handle that, right?
Are you sure he absolutely needs 256gb?
He told me he shouldn't need 256gb, and that if he runs low on storage he will transfer some files too his current PC which has 500gb left of his 1TB HDD (he says he will clean it soon, lol) or our 4TB family NAS, which we use currently only really use for my wife's photos and my kids massive anime collection

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I may have missed this, but has he checked to make certain that what he needs to do in school can be done using OS X? What if the important tasks are in some way Windows-limited? Or if the school servers (if any) aren't OS X friendly?
Yep, they should be, the school uses gives free office 365 for the students and 1TB of cloud storage in onedrive, all of which work with Macs I believe, they also use a fair amount of web apps, which are always multi-platform.
Depending upon where you live in Canada, take a day trip to the states and visit a Apple Store, or other retailer who sells Apple Products. Then when you cross back over the border, just tell them you were sight seeing for the day.
I have friends near Toronto who go to Buffalo all the time to buy products that are so much cheaper in the states.
Take a empty laptop bag with you. Toss all of the box and packing materials in the garbage. Put the new laptop and accessories in the laptop bag. If you get stopped, tell them you had the laptop already when you crossed into the US.
Power on the laptop and do the initial setup before putting in in the bag so that if it is inspected at the border, it will look like it has been used previously. Will be a dead give away if it starts up in new user setup mode!!
No can do, we are in Edmonton so to get to a small town in Montana takes 5+ hours to drive to.
To be honest, once the extended, 3 year warranty has run all bets are off.
If it breaks, the repair will cost about 1/3 or 1/2 of what a new one will cost.
Because they are almost unrepairable in the common sense of the word, in the same way they are not upgradable. Everything is glued or soldered or proprietary.
Of course, you can't say how long it will last.
For every anecdote about some Apple product failing basically day one after the warranty has run out, there's another one about an exact same model doing five or six year heavy-duty work.
The refurbished 21" iMac of my parents lasted something like six years or so and the (very) lightly used MacBookPro we have at work is roughly the same age - but it never leaves the house ;-)
If you can wait, that is always good. More 2013 and 2014 refurbished models will hit the store over the next months, as the 2015 models get sold.
2013 and 2014 models are almost identical (even the GPU is the same).
2015 has a better GPU after all. But the display is still the same.
In the meantime, the new 12" MB (the UltraBook that takes the "Ultra" to new levels) will hit the stores.
I believe, OS X is a nice OS even for a kid. It's sufficiently simply, yet powerful enough so it doesn't get too annoying ;-)
And then there's the Unix foundation beneath the GUI...
The only real problem I could foresee is that your other son will soon want to have his own MacBookAir, too.
So, you might take that into account when you spec out the older's MacBookAir.
I don't have kids, but I know how I thought as a kid ;-)
I can't see myself using an old and slow windows laptop while my brother has an almost surreal, shiny new MBA. At least, I couldn't take that situation for too long.
As mentioned, would it be possible to install an SSD (and more RAM) into the older laptops, so your younger son could use his for some more time?
As for the specs, I don't think 256GB SSD is absolutely necessary - in theory, files that aren't used very often could be stored on an external medium. But 8GB RAM, I'd definitely recommend, if the budget allows it.
11" or 13" is really an individual choice. Make sure your son can test both variants.
First things first, we are not sure even about buying AppleCare, but we may take it into consideration. Second, we most likely are going to wait before we buy them (unless he finds a really good deal), and if we do wait hopefully the Canadian Dollar will raise a bit by then, he also has said that he doesn't like the new macbook calling it "Pretty much just a glorified iPad with a keyboard running OS X." About his brother wanting a MacBook too, he will get that option when he goes into grade 7 next year, I am going to get him a New, thin laptop with good battery life also, since his birthday is in October he can wait a month to save up money and do what my 13 year old is doing, taking the $500-600 that I was originally gonna pay and ad onto it with his own money to get a better one.
I will definitely bring up the idea of paying a bit more for 8gb of RAM.
I also would like to let you guys know that my son is quite good with technology for his age, he basically acted as a unpaid IT for his teachers and had to find a workaround to get all the school computers to work when he was 10, last summer he dual-booted Crunchbang linux on his laptop and installed arch linux in a virtual machine, he also ran a vent and Minecraft server for him and his friends off his laptop in grade 5.