Most of the time a packet of seeds will be way more than the average gardener can plant in a year. The biggest problem when using old seed is when your planting rows, peas are a good example. Many times you will have spots where nothing has grown, but this is not something that is exclusive to old seed, many times new seed will also not germinate very good.

When the seedlings are about a inch tall, use your finger to push in a new seed in the "blank" spots. This is practical for us who have small gardens at home, but would not work on a large scale.