This is a question about polystyrene used for scale models :
before I argue foolishly ignorant ...

UV radiation and cold will definitely embrittle plastic .
But , will plain old time - 10 years - cause the plastic to become brittle ?
Will the solvent adhesive ?

I don't think time by itself - in a 'normal' household environment - would change anything .
and the glue would seem to achieve the opposite , making the joint more plastic .

Not science, just an anecdote -
I have a hardhat I used during the late 1990s, and collected a number of stickers from various projects I worked on then, It's a nice memento of that part of my career.
Hardhats do have a limited lifetime, OSHA requires them to be replaced after 3 years and there is a manuf. date in every hardhat.
OK - so this hard hat I hung up in my garage around 2003. A few months ago I was moving some things around in there and knocked it off its peg. when it hit the floor it shattered into splinters like glass!

Hardhats are made of polyethylene of course, but I think the aging process is likely similar in all cross-linked polymers including styrene.

Keith,
I have several very old kits (like 50+ years old), and a few built models (2 exactly) that I can trace back to 1962. They show no sign of being "brittle". The Monogram 1/4 ton with a 37mm gun was my first kit-- I believe it was manufactured in 1959-- it's unpainted, glue blobs, and still has the hard vinyl tires common to the Monogram military vehicle series of the time. I also have a few old boxed kits-- one, the Revell Mercury Atlas kit made in 1962 is as "new" in the box as it was in 1968 when I bought it for the princely sum of $2.98-- (it took me an entire month to save that up!). So to answer your question I think it depends on several things-- as you said, exposure to UV rays/temperature, purity/quality of the Polystyrene used in the manufacturing process, and perhaps the type of glue/paint used when building the kit. I used to work in a LHS, and a some of the new kits we sold were made in Europe (both east and west), and those kits sometimes had some really brittle plastic in them right in the box, which tended to get worse over time. That said, I have a couple of old Airfix and Heller kits that seem just fine today as they were in the 60s. I do have to say I "updated" an old Aurora Brueget 14 a few years back, and although the model was not "brittle", I couldn't get it to hold any paint on just the wings, even after sanding them down to replace the rib detail. I finally had to resort to a hot lacquer based primer, it seemed the plastic had become incredibly hard and impervious to any surface treatments, even after washing and sanding.
VR, Russ

Thanks for the input , guys

I know it definitely depends on the plastic .

An artist on another forum commented that a build of his was very brittle after 7 years and I asked what caused it . He builds with LOTS of plastic card . I don't know the brand .
His response was the age and the cement .

After researching a bit , I think if your plastic is brittle in a few years in 'household' conditions it was brittle to begin with . or basically cheap-o plastic
I too experimented with some older plastic in the stash -- 25-30 yrs old at least .Revell and Tamiya , and found no brittle components .
Jonathan ,
You made me curious . I went out to the shop and dug out an old hard-hat . A Brown&Root with a Magpie something and some other thing .. One of my dad's , so somewhere mid 80's tops.
It's a cool one . with the perfect peak and brim .
I slammed it against the slab floor a couple of times and it didn't break . Loud though

Like you say , It depends on the quality of the material.
I know MiniArt had a bad run of production using poor styrene , resulting in brittle parts . It was what they could get at the time .
They've relocated and have a new plastic source so they've solved the problem .

I'm still curious about what exactly happens at the glue-joint . If that accelerates the process or the reverse .

Cheers

Keith it would depend a lot on the type of glue, if it's CA (super glue) then the glue itself acts as the bond so it will age and over time weaken. If you use Tamiya super thin or similar from another manufacturer then the way that glue bonds is to melt the plastic and allow it to 'weld' together forming a very strong bond so this type of bond will rely on the plastic.

On the subject of hard hats in the UK the 'life' of the hat is 3 years from the date on the hat unless you're in Scotland then it is 5 years as they get less sun and UV exposure.
I'm a safety rep at work.