Can you play Minecraft mods thats lower then your minecraft version?

I want to add mods into my minecraft soon but i don't know if you can do that. For example if you have a minecraft version of 1.5.1, can you still put in and play mods that are lower versions such as 1.2.5?

You would have to convert over versions, which I would recommend a launcher such as AT Laucher

No. If they could, why would mods need to be updated? Apart from bug fixes