Try using a blunt instrument to probe the cracks and try to open them up, if they do not go deep, or are suface cracks that do not open up as to expose the cords they are ok.

Bear in mind that if you are replacing the two front tyres, you should move the two existing rear ones to the front and fit the two new ones on the rear - it has been shown (at least on fron wheel drive cars anyway) that doing so significantly reduces the chances of such cars losing the back end in wet weather when braking and turning.

This is because its far easier to lose the back end as there's normally far less weight on the rear axle, plus the older, less grippy tyres fitted on that axle. Note that whatever new tyres you go for, its preferable when replacing them that you stick with the same type of tyres all around, i.e. all symmetric (the most basic type of tyre - cheap, but less grip and performance in the wet), asymmetric (they have a completely different tread pattern on the inside and outside of the tyre edge - good for grip, cornering and stability) and directional (great on water dispersion, and grip in snowy conditions for all-season/winter tyres, but a bit less grip than asymmetric and noiser, relatively speaking).

I wouldn't ever mix and match, and you should never do so on the same axle except when using a temporary spare (at lower speeds). Symmetric tyres can be rotated to any corner so are the most versatile, asymetric tyres can be, but you need to make sure the word 'outside' on the tyre sidewall appears on the outside as you llok at it when fitted to the car; directional tyres have an arrow showing the (forward) direction they must be fitted in and can only be rotated (without having to take them off the wheel, refitted and balanced) from front to back. Never buy a direction tyre (fitted to a wheel) as a spare as they can only be fitted to one side of the vehicle.

If you determine (as has been said) that those rears are ok, then I'd follow this advice; if you replace all four, then make sure you rotate the tyres from front to back at least once a year (some say every 5k - 6k) to get an even wear over them all. This is especially important for people like me (though perhaps not for yourself who seem to do a reasonable mileage per year) who do low annual mileages and end up having to change rear tyres that barely have worn (mine are 5-6mm) after 6 or more years.

I prefer, even with advances in tyre technology, to change my car's tyres every 6 years - I may increase this to 7-8 now that my next set will be Michelin CrossClimates that are designed to wear less and perform better than competitors when older/worn, but as I and others have said, the appearance and feel of a tyre when in use is just as much a factor as age - tyres get harder when old, meaning they are less comfortable, give less grip (especially in the wet, snow and ice), are noiser, but actually improve mpg (a bit).

2 years old is fine for age - its the condition you need to check, especially if they've seen a lot of miles, and been drive on lots of poor quality roads with potholes, etc - damage to tyres can easily be a killer, as they can cause blowouts or make loss of control far easier in poor weather. This is especially important if your car has low profile (under 50 profile) tyres, which are much more suscepitable to damage.

Also, as the tyres are your car's only contact with the road, don't settle for cheapo Chinese knock-off tyres - there are some better budget brands around nowadays, but the middle and premium brands are far safer tyres in poor weather and as they wear - a price worth paying for safety. I still find it amazing how many people driving flashy (expensive) cars (even newer ones) use the 'ditch-finder' tyres, often because they didn't appreciate how expensive they are to run when buying the car: low profile premium tyres cost, say £100 - £150 each for an average size car, standard profile [50-65] equivalents from the same brand cost half as much and last a lot longer.

My first OEM (205/55 R16V) set lasted 40k miles (I changed them with 4mm on the fronts and 5-6mm on the rears - they were getting very hard and did not handle well in the wet and were very noisy), my current set are going to be changed at 24k, mainly because they are also 6 years old - the road handling is fine, noise ok, comfort bearable - but the 12yo alloys are slighly mishapen/corroding and as a result the tyres are losing more pressure than they should.

Changing tyres is very subjective in many ways - my new set will be higher profile (195/65 R15H) as my car is allowed these and it gives a smoother ride (I've driven the same model with them and the ride quality was far better [with little difference in handling] than mine even on new tyres), as well as both the replacement alloy wheel and, of course the tyres being far cheaper (tyres 20% less, alloys 25-50% depending on the one selected).

I would say your rears don't look that bad, but that's just my unscientific opinion from looking at the photos. Just make sure they are worn evenly across the inside, centre and outside (measure this yourself at least in three different locations on each tyre), including the ones you're going to replace, as unevenly worn tyres can indicate:

1. Under inflation (more worn edges);
2. Over inflated tyres (centre tread worn more);
3. Inside or outside worn more than eachother by more than 1mm (wheel alingment out).

If that last one applies, then I'd get a full, 4 wheel alignment done, even though it can cost £65 - £100 (or more), so the new tyre arrangement wears evenly across each tyre and you get the best combination of ride quality, grip (safety) and tyre life. If its either of the first two or none, just make sure the tyres are kept inflated to their handbook-recommended levels and keep an eye out for one or more deflating quicker than the others. I would say most new tyres and wheels in good nick should mean that the tyres should lose only 1psi per month, tops, a bit more if the car sits around for long periods (low use).

Best of luck. Sorry about the length of the post.