I feel like it is normal to get some rust, especially over the winter season, where it is wetter and I have been using my vehicle less. Can you tell if they need replacement from the picture? Car is 5 years old and has less than 20k kms.
The shiny rings on the rotors are the only parts that are doing any useful work braking the vehicle .
The dull rings are rusty and are not serving any use. The rust rings will have worn the matching surface of the friction material down on the brake pads, so there will be no friction in these areas.
It looks like there is only about 50% of the surface of the side of the rotor in the photo that is useful and the rusted area will only get bigger. If the other side of the rotor is in the same condition, then this rotor is only providing half of its designed braking performance.
I would suggest that it would be wise to replace the rotors. It is essential to replace the pads at the same time, otherwise the new rotors will look exactly the same in a matter or days.
The image doesn't say much more than "the braking capacity of this wheel is not at 100%." What you need to address is: What is causing this wear pattern? The image suggests there is a problem with the pads or caliper assy. (Not shown in the posted image.)
Do you need a new rotor? The image suggests the rotor will work fine once the real problem is fixed. Will a service station tell you that you need a new rotor. Yes (See @Martin)
I am unsure about the rotors, but a brake overhaul is definitely needed.
Most of the time the mechanics decide that a job isn't complete, unless everything got replaced: either because they want more money or the fear about the customer coming back and complaining...
Also in defense of those mechanics out there: Cost of rotors is quite low, relatively to the hourly rates. So when everything was taken apart, cleaned and lubed, it makes sense to replace the rotors as well if they don't look pristine.
Front or rear rotors? Because the rears do very little braking and often end up like this anywhere there's moisture or salt. If you are too gentle the fronts can too.
As long as there's enough thickness of rotor left, try some (SAFE) hard stops from a decent speed to clean the rotors, often that's all they need.
I usually do this if I'm coming off the motorway on a slip road and there's no-one behind me I'll give the brakes a really good hard push just to knock the rust off the rears.
The Shiny ring should be one thick unbroken ring the width of the brake pad. There's something wrong with the pad or calliper because it's not normal to have a rusty ring in the middle of two shiny rings.
This means the pad is not contacting the rotor properly and it's leaving a gap. I agree with the mechanic. Since the shiny ring is what's worn and if you glide your fingernail over the rings you'll likely find it's higher on the rusty ring in the middle. Not changing the rotors will cause the new pads to not bed in properly and wear unevenly.