What are the limitations to an outdated backup? My backup private keys were made in Jan and my computer crashed in May. If I upload the keys on a new wallet, will it show my bitcoin balance from May or January?
Bitcoin-Qt has a feature called "keypool." It pre-generates 100 keys and includes them in all of your backups. This way, if you make a backup, it will also include the keys you're going to use in the future.

In general, sending a payment consumes an address, and receiving a payment consumes an address.

If you make 1 transaction a month, 100 keys will last 8 years.

However, if you change your wallet password, you must make a new backup.