I have an iPhone 3G. I can't download any apps. My latest update is 4.2.1?

And 4.2.1 is the higest on my phone. Anyone know why. Please help. Will it help with I jail break it. I don't know.

You really think you can put new software on an old iPhone 3G? Come one dude, don't be stupid. Do yourself a favor and go buy a newer phone. You can find cheep ones too so there's absolutely no excuse.

The iOS limit on your device is mostly due to the type of CPU and RAM memory in the device.
iTunes promotes the latest greatest apps that run on iOS 5 or later. Try,
https://sites.google.com/site/appleclubfhs/support/advice-and-articles/finding-apps-for-older-devices
or
https://itunes.apple.com/...?mt=8&uo=4
(You still will not be able to get apps that have been updated by the developer past the maximum iOS version in your device)