7 years old? You may have some problems. I am going to assume (yeah, yeah, I know what they say) that you use your middle chainring more than the small and large, therefore it is worn more. You can replace the middle chainring by itself provided if it is not on a real low end bike where all 3 chainrings are attached as one unit. Here is where your problems may come in: chains stretch - quite a bit, especially after 7 years. Your chainrings and cogs (rear gears) have worn with the stretched chain. So, if you put a new chainring on and don't replace anything else, the middle chainring will not mesh properly with the old chain and guess what - it will skip also! Seems weird, but it happens. Usually when this happens it is necessary to replace all chainrings, cogs, and chain. Very expensive. Your least expensive bet may be to try and find a used chainring that has some life in it.

It is a little late for this advice, but it may help in the future: Replace your chain often. It will extend the life of your other more expensive drivetrain components. How often depends on how often you ride. I usually replace mine about 3x year on a MTB that probably sees 1800-2000 trail miles/year on it.

One other note, if you have to replace all of your drivetrain parts, it is usually cheaper to buy a whole new set of shimano cranks with the chainrings than to buy the individual chainrings.

Good luck.