Hi,
I don't know how true this or not, but I heard if you have seeds from the previous year and you plant them the next year, they're not viable because of them being from an older batch of seeds. Is this true? ~ No, most seeds last a few years if not longer. Only a few have a short window of viability. You do need to store them properly in a cool, dark, dry place.

Do older seeds from the previous year affect their vitality in some shape or form? ~ No, most will be the same in year 2.

Does it make them more susceptible to disease or problems? ~ No.

If it's not true that how old a seed doesn't effect it's vitality how long can you store seeds from a previous batch and previous year without their vitality being effected?
(1 year, 2 years, etc.) ~ So much depends on what seed it is. There are way too many plants make a list. It is better if you ask about specific seeds. There are many posts. You can do a search, and if you don't find an answer, ask : )
Remy

Age affects the germination rate of seeds, it diminishes some every year, faster for some than others. If seeds do germinate, the age of the seed does not affect how it will grow, the health or the productiveness of the plant. Seeds should be kept in a cool, dark, dry place, and never allowed to get damp. I keep mine in a container in the refrigerator.

Good gardening, Mary

No, it's not true. Some seeds do last longer than others, and it is best to keep them dry and cool. But most seeds should be fine for at least 3 years. Many varieties longer than that. Leeks are supposed to be among the shortest-lived seeds, but I planted some from last year and they sprouted just as well as the new ones.
I used to buy new seeds every year, but no more.

I planted some acorn squash seeds from 2004 (they were kept in a dark closet in the basement), and even though they took longer than my newer seeds, about 2/3 of them germinated and are now happy little seedlings in my seed starting trays. 2 years ago I used the first batch from that group, and every seed sprouted, so I did lose about 30% viability, but from 2004 to 2007, I did not lose anything. Give it a try as long as they're in a dark, cool place.