Tu vs Vous
What is the difference between Tu and Vous?
Tu is singular for you. So you would use tu (and all corresponding conjugations of verbs) if you are speaking to one person. Vous is plural for you, so you would usually use vous (and all corresponding conjugations of verbs) when speaking to a group of two or more people. However, here’s where it gets a bit tricky, vous is also used as the more formal way to adress a person. If you are being more formal, or showing respect (someone older, or a superior at work for example) you would address the person as vous.
Do you have to say 'vous' to a person that is like, say a day older than you or would it still be 'tu'?
If you consider them a peer then you would say tu. So if they are a day older than you would say tu. However, if you don’t know them very well you might choose to say vous to be extra polite. Or if they are your age but they are your boss at work, or if they are royalty, you would say vous. Most cases though, if you are on a first name basis with them, I think it is safe to say tu. I am not french though, so a francophone might have a better answer. Here is a flowchart http://frenchyourway.com.au/tu-or-vous but don’t worry too much about memorizing all the rules! I imagine even french people get confused by them sometimes, it is just something fun to look at. Just remember friend/peer: tu, someone who you want to show more formal respect to: vous.