I'm often asked in canning classes how I feel about using old jars. Really depends on what you classify as old. I've had several students so worried about food safety that they never used a jar twice! On the other hand, I've had students use the oldest, narliest jars (including mayo jars) and they can't figure out why their canning success ratio is so low!
Canning jars are made to be used again and again. The glass is very thick and can withstand the heat of a water bath canner or a pressure canner. That's why it's not safe to use a jar from the store that you purchased spaghetti sauce or mayo in. These jars are very rarely thick enough for your purpose. The process used to store foods in these containers does not require a sturdy jar that can withstand pressure and heat.
However, even a canning jar can get worn out.
Always check your jars for even the tiniest chips in their rims. An uneven rim prohibits a lid from pressing tightly against the jar. Beautiful old jars are usually best for decoration only!
Try this: take a new jar and an old jar, and run your fingertip lightly around the rims of both, and you'll easily feel the difference. Old jars were manufactured with less accurate technologies, and their rims are frequently uneven and often sport a raised ridge on the inside rim. Food canned in older jars has a higher rate of improper sealing because the lid cannot snugly connect to a faulty rim. Even if a jar seems sealed today, an older rim may be struggling to stay sealed, and in the middle of winter you may find your once-sealed jar to be floating in moldy scum!
Older jars can still be used to store dry ingredients, seeds, or just look beautiful on a windowsill! New jars can be used for quite a long time, so you'll get your money's worth, AND you will have safe food to serve your family and friends!