My 13 year old maytag refrigerator with top freezor is not cooling enough. Generally is it worth repairing or should I buy new?

I assume its been cleaned out of dust buildup and defrosted.. and still does this.

Opinion... if it's that old or older... I'd buy a new one.

The problem could be that it needs the coils washed out under the unit, or something simple like that. Generally, I would advise fixing it unless it is something like a compressor and the rest of it is in bad shape, but I have replaced many compressors in these units with good results. Almost all problems are due to dirt in the coils, at least that starts the problems. People don't realize that they need to clean the coils regularly or it will stop cooling. Wash out the coils by removing rear cover (replace before turning back on.) and spray the coils and fan blades down with Fantastic, 409, or another good cleaner, then wash off with sprayer on water hose until clean. Let dry, clean rear cover, and replace it. You will also want to pull out drain pan and clean it, too. This may cure it.

Personally, it may end up more cost effective to just buy a new one. Plus you could run into issues of certain parts not being manufactured for your current fridge anymore.

First it is hard to judge because we do not know how much it may cost to repair,

In general one that is 13 years old is not as energy efficient as one that is made now. And of course it last for a long time. At 13 years personally I would ( if I could afford to buy one) get a new one. But if buying is not a real good option, I would try some of the ideas above

I disagree on the energy efficiency. If it has R-12 as a refrigerant it is 50% more energy efficient than the new models with R-134a. R-12 and the older refrigerants are 50-60% more energy efficient than the newer gases. The new gases were forced on us by the leftists to cripple us energy-wise and have reduced the gas mileage on our cars and driven our electric bills up. Shortly after the law was passed with the EPA it was discovered that the "ozone hole" was a natural occurrence due to the tilt of the earth for two weeks and not because of the "5 times heavier than air" freons. We need to lobby for the repeal of these laws.